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Protocols for Radon and Radon Decay Product Measurements in Homes 

Office of Air and Radiation (6609J) 

EPA 402-R-93-003, June 1993 

Preface 

This document, the Protocols for Radon and Radon Decay Product Measurements in Homes (EPA 

402-R-92-003, May 1993), is a guidance document. However, one condition of participation in the 

Agency's National Radon Proficiency Program (RPP) for radon measurement and radon reduction 

(mitigation) proficiency, is conformance with these protocols. Conformance with its companion 

document, the Indoor Radon and Radon Decay Product Measurement Device Protocols (EPA 402-R92-004, July 1992), is also a condition of participation in the Proficiency Program. 

Together these protocol documents provide the technical support for the Agency's radon policy and 

guidance to consumers that is contained in, but not limited to, the Home Buyer's and Seller's Guide to 

Radon (EPA 402-R-93-003, March 1993), A Citizen's Guide to Radon (EPA 402-K-92-001), and the 

Consumer's Guide to Radon Reduction (EPA 402-K-92-003, August 1992). 

Table of Contents

Section 1: Introduction

Section 2: Discussion of Guidelines Presented in the Citizen's Guide to Radon

2.1 Introduction and Summary

2.2 Measurement Location

2.3 Initial Measurements

 2.3.1 Rationale 

 2.3.2 Closed-Building Conditions 

 2.3.3 Interpretation of Initial Measurements Results 

2.4 Follow-Up Measurements

 2.4.1 Rational 

 2.4.2 Short-Term and Long-Term Follow-Up Testing 

Section 3: Discussion of Guidelines Presented in the Home Buyer's and Seller's Guide to Radon 

3.1 Introduction

3.2 Options for Real Estate Testing

 3.2.1 Option 1: Sequential Testing 

 3.2.2 Option 2: Simultaneous Testing 

 3.2.2.1 Both measurement Results Equal to or Great than 4 pCi/L 

 3.2.2.2 Both Measurements Result Less Than 4 pCi/L 

 3.2.2.3 One Measurement Results Greater Than 4pCi/L and One Measurement Results 

 Less Than 4pCi/L 

 3.2.2.4 Precision Requirements 

 3.2.2.5 recommended Language fir Informing the Client that a Retest is Warranted 

 3.2.3 Option 3: Active Monitor Testing 

3.3 Measurement Location
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3.4 Measurement Checklist

3.5 Interference-Resistant Testing

 3.5.1 Influencing Test Area Concentration 

 3.5.2 Equipment Interference 

 3.5.3 Preventing Interference 

 3.5.4 Interference-Resistant Detectors 

Section 4: General Procedural Recommendations

4.1 Introduction

4.2 Initial Client Interview

4.3 Measurement Recommendations

 4.3.1 Selecting a Measurement Approach 

 4.3.2 Written Measurement Guidance 

 4.3.3 Conditions for a Valid Measurement 

 4.3.4 Non-Interference Controls 

 4.3.5 Measurement Documentation 

4.4 Quality Assurance in Radon Testing

 4.4.1 Calibrate Measurements 

 4.4.2 Known Exposure Measurements 

 4.4.3 Background Measurements 

 4.4.4 Duplicate Measurements 

 4.4.5 Routine Instrument Measurement 

 4.4.6 Quality Assurance Plans 

4.5 Standard Operating Procedures

4.6 Providing Information to Consumers

4.7 Reporting Test Results

4.8 Temporary Risk Reduction Measures

4.9 Recommendations for Mitigation

4.10 Worker Safety

Appendix A: State and EPA Regional Radon Offices

Appendix B: Interpretation of the Results of Simultaneous Measurements

 B.1 Assessment of Precision 

 B.2 Example Controls Charts for Precision 

 B.2.1 Sequential Control Chart Based on Coefficient on Verification 

 B.2.2 Sequential Control Chart Based on Relative Percent Difference 

 B.2.3 Range Control Chart 

 B.3 Interpretation of Precision Control Chart 

Glossary

References
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Section 1: Introduction 

This document presents the U.S. Environmental Protection Agency's (EPA) technical guidance for 

measuring radon concentrations in residences. It contains protocols for measuring radon for the 

purpose of deciding on the need for remedial action, as presented in the 1992 Citizen's Guide to Radon

(EPA 402-K-92-001; U.S. EPA 1992a), and in the Home Buyer's and Seller's Guide to Radon (EPA 

402-R-93-003; U.S. EPA 1993). 

The guidance for determining the need for mitigation is different in several key aspects from previously 

issued recommendations, and this document supersedes a previous report (EPA 520/1-86-014-1) 

published in February, 1987 (U.S. EPA 1987). The technical basis for these policy changes is supplied 

in the Technical Support Document for the 1992 Citizen's Guide to Radon (EPA 400-R-92-011; U.S. 

EPA 1992g), and the revised policies are described in Section 2 of this report. 

Section 3 of this report describes the Agency's recommended protocols for measuring radon for a real 

estate transaction. This guidance elaborates on Agency recommendations published in the Home 

Buyer's and Seller's Guide to Radon (EPA 402-R-93-003; U.S. EPA 1993). The radon testing 

guidelines in the Home Buyer's Guide were developed specifically to deal with the time-sensitive nature 

of home purchases and sales and the potential for radon device interference. The guidelines are 

somewhat different from those in other EPA publications, such as the 1992 Citizen's Guide to Radon

(EPA 402-K-92-001; U.S. EPA 1992a), which provide radon testing and reduction information for nonreal estate situations. Therefore, Sections 2 and 3 of this document will have different guidance for 

different situations. 

This report is limited to discussions of Agency guidance regarding detector placement, measurement 

duration, multiple measurements, and the interpretation of measurement results. EPA has also issued a 

technical report describing measurement techniques, titled Indoor Radon and Radon Decay Product 

Measurement Device Protocols (EPA 520-402-R-92-004) and published in 1992 (U.S. EPA 1992c). 

That report provides technical information for measuring radon concentrations with continuous radon 

monitors, alpha track detectors, electret ion chambers, charcoal canisters, unfiltered alpha track 

detectors, and grab radon techniques; it also provides guidance for measuring radon decay product 

concentrations with continuous working level monitors, radon progeny integrating sampling units, and 

grab radon decay product techniques. Copies of the Indoor Radon and Radon Decay Product 

Measurements Device Protocols may be obtained by contacting your State or EPA Regional radon 

office (Appendix A) A list of EPA documents providing guidance on radon measurements appears in 

Exhibit 1-1 

* The term "radon" refers to radon-222 and its decay products unless otherwise noted. 
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Title of Document EPA Document Number 

A Citizens Guide to Radon 

(U.S. EPA 1992a) 

EPA 402-K-92-001 

Consumer's Guide to Radon Reduction 

(U.S. EPA 1992b) 

EPA 402-K-92-003 

Indoor Radon and Radon Decay Product 

Measurements Device Protocols 

(U.S. EPA 1992c) 

EPA 520-402-R-92-004 

Interim Radon Mitigation Standards 

(U.S. EPA 1992d) 

Regional Training Centers 

(see below) 

Home Buyer's and Seller's Guide to Radon 

(U.S. EPA 1993) 

EPA 402-R-93-003 

Protocols for Radon and Radon Decay Product 

Measurements in Homes 

EPA 402-R-92-003 

*These documents are available from the U.S. Government Printing Office, Superintendent of 

Document, Mail Stop: SSOP, Washington, D.C. 20402-9328; from the National Technical Information 

Service, U.S. Department of Commerce, Springfield, Virginia 22151; or your State or EPA Regional 

Radon office. 

EPA Regional Radon Training Centers:

Eastern Regional Radon Training Center, Rutgers University; (908)-932-2582. 

Southern Region Radon Training Center, Auburn University; (205)-844-6271. 

Western Regional Radon Training Center, Colorado State University; (303)-491-7742 

Northern Regional Radon Training Center, University of Minnesota; (612)-624-6786 

This report provides guidelines that are primarily intended to aid State radiation control programs, other 

organizations conducting indoor radon measurements, and homeowners who want detailed information 

on radon measurements. The guidelines herein can be adopted as part of a State program or can be 

provided by States to interested individuals as recommendations. Adherence to these guidelines was a 

requirement for participation in EPA's former National Radon Proficiency Measurement Proficiency 

(RMP) Program (EPA 520/1-91-006; U.S. EPA 1991). The method designations that were used in 

EPA's former RMP listed in Exhibit 1-2. A two-letter code for each method has been adopted, although 

ATDs (AT), RPISUs (RP), and EICs/ECs (ES or EL) may still be referred to by their traditional 

acronyms. 

EPA recognizes that radon concentrations in buildings may vary over time (Arvela et.al. 1988, Dudney 

et.al. 1990, Fleischer and Turner 1984, Furrer et.al. 1991, Gesell 1983, Harley 1991, Hess 1985, Martz 

et.al. 1991, Nyberg and Bernhardt 1983, Perritt et.al. 1990, Ronca-Battista and Magno 1988, Steck 
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1992, Stranden et.al. 1979, Wilkening and Wicke 1986, Wilson et.al. 1991). Furthermore, 

concentrations at different locations in the same house often vary by a factor of two or more (Arvela 

et.al. 1988, Furrer et.al. 1991, George et.al. 1984, Hess 1985, Keller et.al. 1984, Put and deMeijer 

1988, Steck 1992). EPA has carefully evaluated these findings, as well as other factors (EPA 400-R-92-

011; U.S. EPA 1992g), and has developed policies for ensuring that the most representative and useful 

information is supplied by the measurement results. These guidelines may be evaluated periodically 

and refined to reflect the increasing knowledge of, and experience with, indoor radon. 

EPA recommends that initial measurements be short-term tests and performed under closed-building 

conditions. An initial short-term test, which lasts for two to 90 days, ensures that residents are informed 

quickly should a home contain very high radon levels. Long-term tests, give a better estimate of the 

year-round average radon level. The closer the long-term test is to 365 days, the more representative it 

will be of annual average radon levels. 

Exhibit 1-2 

Radon and Radon Decay Product Measurement Method Abbreviations 

Abbreviations

METHOD CATEGORY Common Method

Continuous Radon Monitors CRM CR 

Alpha Track Detectors ATD AT 

Electret Ion Chambers 

Short Term 

Long Term 

EIC/EC 

ES 

EL 

Activated Charcoal Adsorption Devices 

(formerly called charcoal canisters) 

CC AC 

Charcoal Liquid Scintillation CLS LS 

Three-day Integrating Evacuated Scintillation Cells SC 

Pump/Collapsible Bag Devices 

(24 hour sample) 

 PB 

Grab Radon Sampling 

 Scintillation Cells 

 Activated Charcoal 

 Pump-Collapsible Bag 

 

GS 

GC 

GB 

Unfiltered Track Detectors UTD UT 

Continuous Working Level Monitors CWLM CW 

Radon Progeny Integrating Sampling Units RPISU RP 

Grab Sampling - Working Level GW 
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Section 2: Discussion of Guidelines Presented in EPA's "A Citizen's Guide to Radon" 

2.1 Introduction and Summary

2.2 Measurement Location

2.3 Initial Measurements

2.4 Follow-Up Measurements

2.1 Introduction and Summary 

The Citizen's Guide to Radon ( EPA 402-K-92-001; U.S. EPA 1992a)presents a measurement strategy 

for assessing radon levels in homes for the purpose of determining the need for remedial action. This 

measurement strategy is intended to reduce the risk to public health from exposure to radon in air in 

homes. The strategy begins with an initial measurement made to determine whether a home may 

contain radon concentrations sufficient to cause high exposures to its occupants. 

EPA recommends that initial measurements be short-term tests placed in the lowest lived-in level of the 

home, and performed under closed-building conditions. An initial short-term test ensures that residents 

are informed quickly should a home contain very high levels of radon. Short-term tests are conducted 

for two days to 90 days. Closed-building conditions (Section 2.3.2) should be initiated at least 12 hours 

prior to testing for measurements lasting less than four days, and are recommended prior to tests 

lasting up to a week. 

If the short-term measurement result is equal to or greater than 4 picoCuries per liter (pCi/L), or 0.02 

working levels (WL), a follow-up measurement is recommended. Follow-up measurements are 

conducted to confirm that radon levels are high enough to warrant mitigation. If the result of the initial 

measurement is below 4 pCi/L, or 0.02 WL, a follow-up test is not necessary. However, since radon 

levels change over time, the homeowner may want to test again sometime in the future, especially if 

living patterns change and a lower level of the house becomes occupied or used regularly. 

There are two types of follow-up measurements that may be conducted, and the choice depends, in 

part, on the results of the initial test. An initial measurement result of 10 pCi/L (or 0.05 WL) or greater 

should be followed by a second short-term test under closed-building conditions. If the result of the 

initial measurement is between 4 pCi/L (or 0.02 WL) and 10 pCi/L (or 0.05 WL), the follow-up test may 

be made with either a short-term or a long-term method. Long-term tests are conducted for longer than 

90 days, and give a better estimate of the year-round average radon level. The closer the long-term 

measurement is to 365 days, the more representative it will be of annual average radon levels. On the 

other hand, short-term tests yield results more quickly and can be used to make mitigation decisions. If 

the long-term follow-up test result is 4 pCi/L, or 0.02 WL, or higher, EPA recommends remedial action. 

If the average of the initial and second short-term results is equal to or greater than 4 pCi/L, or 0.02 WL, 

radon mitigation is recommended. These recommendations are summarized in Exhibit 2-1. 
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Exhibit 2-1

In certain instances, such as may occur when measurements are performed in different seasons or 

under different weather conditions, the initial and follow-up tests may vary by a considerable amount. 

Radon levels can vary significantly between seasons, so different values are to be expected. The 

average of the two short-term test results can be used to determine the need for remedial action. 

The testing strategy policies presented here allow homeowners to decide on the need for mitigation 

with a high level of confidence that their decision is correct (EPA 400-R-92-011; U.S. EPA 1992g). 
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2.2 Measurement Location 

Short-term or long-term measurements should be made in the lowest lived-in level of the house. The 

following criteria should be used to select the location of the detectors within a room on this level: 

• The measurements should be made in the lowest level which contains a room that is used 

regularly. Test areas include family rooms, living rooms, dens, playrooms, and bedrooms. A 

bedroom on the lower level may be a good choice, because most people generally spend more 

time in their bedrooms than in any other room in the house (Chapin 1974, Moeller and Underhill 

1976, Szalai 1972). If there are children in the home, it may be appropriate to measure the 

radon concentration in their bedrooms or in other areas where they spend a lot of time, such as 

a playroom, that are situated in the lowest levels of the home. 

• In general, measurements should not be made in kitchens, laundry rooms, or bathrooms. The 

measurements should not be made in a kitchen because of the likelihood that an exhaust fan 

system and changes in small, airborne particles (caused by cooking) may affect the stability of 

WL measurements. Measurements should not be made in a bathroom because relatively little 

time is spent in a bathroom, because high humidities may affect the sensitivity of some 

detectors, and because of the likelihood that use of a fan may temporarily alter radon or decay 

product levels. 

Although radon in water may be a contributor to the concentration of airborne radon, radon in air 

should be measured before any diagnostic radon-in-water measurements are made. (Diagnostic 

measurements may be made in the bathroom; however, such diagnostic measurements should 

not be used to determine the need for mitigation.) 

• A position should be selected where the detector will not be disturbed during the measurement 

period and where there is adequate room for the device. 

• The measurement should not be made near drafts caused by heating, ventilating and air 

conditioning vents, doors, fans, and windows. Locations near heat, such as on appliances, near 

fireplaces or in direct sunlight, and areas of high humidity should be avoided. 

• Because some detectors are sensitive to increased air motion, fans should not be operated in 

the test area. Forced air heating or cooling systems should not have the fan operating 

continuously unless it is a permanent setting. 

• The measurement location should not be within 90 centimeters (three feet) of the doors and 

windows or other potential openings to the outdoors. If there are no doors or windows to the 

outdoors, the measurement should not be within 30 centimeters (one foot) of the exterior wall of 

the building. 

• The detector should be at least 50 centimeters (20 inches) from the floor, and at least 10 

centimeters (four inches) from other objects. For those detectors that may be suspended, an 

optimal height is in the general breathing zone, such as two to 2.5 meters (about six to eight 

feet) from the floor. 

Sound judgment is required as to what space actually constitutes a room. Measurements made 

in closets, cupboards, sumps, crawl spaces, or nooks within the foundation should not be used 

as a representative measurement. 
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2.3 Initial Measurements 

2.3.1 Rationale

EPA recommends that a homeowner assessing the need for mitigation should first make a short-term 

test. Short-term measurements can be simple, produce results quickly, and allow the public to make 

decisions about radon reduction that are cost-effective and protective of human health. 

The duration of short-term measurements can range from 48 hours to 90 days, depending upon the 

method used. 

2.3.2 Closed – Building Conditions

Short-term measurements lasting between two and 90 days should be made under closed-building 

conditions. Closed-building conditions are necessary for short-term measurements in order to stabilize 

the radon and radon decay product concentrations and increase the reproducibility of the 

measurement. Windows on all levels and external doors should be kept closed (except during normal 

entry and exit) during the measurement period. Normal entry and exit include a brief opening and 

closing of a door, but--to the extent possible--external doors should not be left open for more than a few 

minutes. In addition, external-internal air exchange systems (other than a furnace) such as highvolume, whole-house and window fans should not be operating. However, attic fans intended to control 

attic and not whole building temperature or humidity should continue to operate. Combustion or makeup air supplies must not be closed. 

In addition to maintaining closed-building conditions during the measurement, closed-building 

conditions for 12 hours prior to the initiation of the measurement are a required condition for 

measurements lasting less than four days, and are recommended prior to measurements lasting up to 

a week in duration. Normal operation of permanently installed energy recovery ventilators (also known 

as heat recovery ventilators or air-to-air heat exchangers) may also continue during closed-building 

conditions. In houses where permanent radon mitigation systems have been installed, these systems 

should be functioning during the measurement period. 

Closed-building conditions will generally exist as normal living conditions in northern areas of the 

country when the average daily temperature is low enough so that windows are kept closed. Depending 

on the geographical area, this can be the period from late fall to early spring. In some houses, the most 

stable radon levels occur during late fall and early spring, when windows are kept closed but the house 

heating system (which causes some ventilation and circulation) is not used. Available information about 

variations of indoor radon levels in a particular area can be used to choose a measurement time when 

the radon concentrations are most stable. 

It may be necessary, however, to make measurements during mild weather, when closed-building 

conditions are not the normal living conditions. It will then be necessary to establish some more 

rigorous means to ensure that closed-building conditions exist prior to and during the measurements. 

Those performing measurements in southern areas that do not experience extended periods of cold 

weather should evaluate seasonal variations in living conditions and identify if there are times of the 

year when closed-building conditions normally exist. Ideally, measurements should be conducted 

during those times. The closed-building conditions must be verified and maintained more rigorously 

when they are not the normal living conditions. Air conditioning systems that recycle interior air can be 

operated during the closed-building conditions when radon measurements are being made. However, 

homeowners should be aware that any air circulation system can alter the radon decay product 

concentration without significantly changing the radon concentration. 
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Short-term tests lasting just two or three days should not be conducted during unusually severe storms 

or periods of unusually high winds. Severe weather will affect the measurement results in several ways. 

First, a high wind will increase the variability of radon concentration because of wind-induced 

differences in air pressure between the building interior and exterior. Second, rapid changes in 

barometric pressure increase the chance of a large difference in the interior and exterior air pressures, 

consequently changing the rate of radon influx. Weather predictions available on local news stations 

can provide sufficient information to determine if these conditions are likely. While unusual variations 

between radon measurements may be due to weather or other effects, the measurement system 

should be checked for possible problems. 

During any short-term test, closed-house conditions should be maintiained as much as possible while 

the test is in progress. In test lasting less than four days (96 hours), closed-house conditions should be 

maintained for at least 12 hours before conditions should be maintained while the test is in progress; 

while recommended, the 12 hour closed-house conditions should be maintained as much as possible 

while the test is in progress. 

2.3.3 Interpretation of Initial Measurement Results

If the initial measurement result is less than 4 pCi/L, or 0.02 WL, follow-up measurements are probably 

not needed. There is a relatively low probability that mitigation is warranted if the result is less than 4 

pCi/L or 0.02 WL (EPA 400-R-92-011; U.S. EPA 1992g). Even if the measurement result is less than 4 

pCi/L, or 0.02 WL, however, a homeowner may want to test again sometime in the future. If the 

occupants' living patterns change or renovations are made to the home and they begin using a 

lower level (such as a basement) as a living area, a new test should be conducted on that level.

The average year-round residential indoor radon level is estimated to be about 1.3 pCi/L, and about 0.4 

pCi/L of radon is normally found in outside air. The U.S. Congress has set a long-term goal that indoor 

radon levels be no more than outdoor levels. There is some risk from radon levels below 4 pCi/L, and 

EPA recommends that the homeowner consider reducing the radon level if the average of the first and 

second short-term measurements or if a long-term follow-up measurement is between 2 and 4 pCi/L 

(0.01 and 0.02 WL). While it is not yet technologically achievable for all homes to have their radon 

levels reduced to outdoor levels, the radon levels in some homes today can be reduced to 2 pCi/L or 

below. 

If the result of the short-term measurement is equal to or greater than 4 pCi/L, or 0.02 WL, the occupant 

should conduct a follow-up measurement using a short-term or long-term test, as described in Section 

2.4.

2.4 Follow-Up Measurements 

2.4.1 Rationale

The purpose of a follow-up measurement is to provide the homeowner with enough information to make 

an informed decision on whether to mitigate to reduce radon levels. The follow-up measurement, 

whether it is short-term or long-term, provides an additional piece of information to confirm that radon 

levels are high enough to warrant mitigation. There are two major reasons why a second measurement 

is necessary. First and most important, radon levels fluctuate over time (see Section 1), and a second 

short-term measurement, when averaged with the first test result, will provide a more representative 

value for the average radon level during the period of the test. If a long-term follow-up measurement is 

conducted, that result should provide an even more representative value for the long-term average 

radon concentration. The second reason for making a follow-up measurement prior to mitigation is that 

there is a small chance of laboratory or technician error in all measurements, including radon 

measurements, and a second test will serve as a check on the first. 
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Homes tested using the protocol in this section should not be mitigated on the basis of a single shortterm test. A follow-up test is necessary for mitigation decision-making regardless of the initial test result. 

2.4.2 Short-Term and Long-Term Follow-Up Testing

Follow-up testing should be conducted in the same location as the first measurement (see Section 2.2). 

A follow-up test can be conducted with either a short-term or long-term measurement device. Longterm tests (> 90 days) will produce a reading that is more likely to represent the home's year-round 

average radon level than a short-term test. However, if the initial test result is high (for example, greater 

than about 10 pCi/L, or 0.05 WL) or if results are needed quickly, EPA recommends a second shortterm test. This will allow the homeowners to obtain information necessary to decide quickly on the need 

for mitigation. If the result of the initial measurement is between 4 pCi/L and 10 pCi/L (or between 0.02 

WL and 0.05 WL), then either a short-term or long-term test can be taken. 

If the long-term follow-up test result is 4 pCi/L, or 0.02 WL, or higher, then EPA recommends remedial 

action. Likewise, if the average of the initial and second short-term results is equal to or greater than 4 

pCi/L, or 0.02 WL, radon mitigation is recommended. These recommendations are summarized in 

Exhibit 2-1. 

As with the initial short-term test, the second short-term test should be conducted under closed-building 

conditions (Section 2.3.2). These conditions, however, are not necessary for long-term tests (those 

lasting longer than 90 days). 
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Section 3: Discussion of Guidelines Presented in The Home Buyer's and Seller's Guide to 

Radon

3.1 Introduction

3.2 Options for Real Estate Testing

3.3 Measurement Location

3.4 Measurement Checklist

3.5 Interference-Resistant Testing

3.1 Introduction 

The unique nature of a real estate transaction, involving multiple parties and financial interests, 

presents radon measurement issues not encountered in non-real estate testing. EPA's objectives for 

issuing recommended protocols for radon measurements made for real estate transactions are 

intended to reduce misunderstanding and protect public health in several ways. First, EPA seeks to 

provide home buyers, sellers, real estate agents, and testing organizations with a common basis of 

understanding of the recommended procedures for radon measurements. Second, the widespread 

implementation of this guidance will produce results that are reliable indicators of the need for 

mitigation. A significant proportion of radon measurements are conducted as part of real estate 

transactions, and all aspects of these transactions are carefully scrutinized, so specific guidance from 

EPA can help to ensure good quality measurements. When the results are interpreted properly and the 

appropriate remedial action is taken, these protocols will assist the buyer and seller in reducing the risk 

to the occupants from radon exposure. The availability of a nationally-recognized protocol for 

measurement and for the interpretation of the measurement results will greatly assist home buyers, 

sellers, real estate agents, builders, lenders, and radon measurement experts. 

These protocols are designed for use in residences, as described in the EPA document, Home Buyer's 

and Seller's Guide to Radon (EPA 402-R-93-003; U.S. EPA 1993). While that document offers general 

information on radon and testing, this report presents a more technical description of EPA 

recommendations, including discussion of guidelines for the interpretation of measurement results. As 

with all of EPA's policies regarding radon measurements, these guidelines have been developed after 

review and assistance from the radon measurement community and EPA's Science Advisory Board. 

Technical information on a variety of radon measurement methods is available in the EPA report titled 

Indoor Radon and Radon Decay Product Measurement Device Protocols (EPA 520-402-R-92-004; 

EPA 1992c; these and other EPA publications are available at this web site or from your State or 

regional EPA office, see Appendix A). 

The radon testing guidelines in the Home Buyer's and Seller's Guide to Radon have been developed 

specifically to deal with the time-sensitive nature of home purchases and sales. These guidelines are 

somewhat different from the guidelines in other EPA publications, such as the 1992 Citizen's Guide to 

Radon (EPA 402-K-92-001; U.S. EPA 1992a), which provide radon testing and reduction information 

for non-real estate situations. 

There are also guidelines in the Home Buyer's and Seller's Guide to Radon to deal with the potential for 

rason test interference. There are approaches that can be used to increase confidence in measurement 

results by detecting measurements interference. For example, a device that offers a variety of ways to 

detect tampering may serve to deter as well as detect interference with the device's operation or with 

proper closed-building measurement conditions. Potential tampering indicators include the ability to a 

device to record changes in radon levels, temperature, and humidity, or to detect movement of or 

around the device during the measurements. Refer to section 3.5 for information and recommendations 

for the interference-resistant testing. 
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EPA investigated a variety of options for real estate testing. EPA recommends testing in advance of 

putting the house on the market. A long-term test, which is conducted for longer than 90 days, gives the 

most representative indication of the annual average radon concentrations in a home. However, for 

time-sensitive real estate transactions, the Home Buyer's Guide offers three short-term testing options. 

Short-term tests are conducted from two days to 90 days, depending on the measurement device. 

Based on extensive quantitative analyses to evaluate the frequency with which long-term and shortterm testing results lead to the same mitigation decision, EPA and its independent Science Advisory 

Board concluded that short-term tests can be used to assess whether a home should be remediated. 

The reliability of each radon measurement made for a real estate transaction, or for any purpose, is 

highly dependent upon the existence and documentation of an adequate quality assurance program 

implemented by both the tester and the analysis laboratory. All the parties involved in the real estate 

transaction depend upon the testers doing their job. This includes ensuring that the measurements are 

valid via the performance of quality control measurements and activities, and detecting measurement 

interference. The protocols outlined in this section were developed by EPA for testers and homeowners 

adhering to the quality assurance practices summarized in Section 4.4 of this report, and in EPA's 

Indoor Radon and Radon Decay Product Measurement Device Protocols (EPA 520-402-R-92-004; U.S. 

EPA 1992c). 

Three options were determined to be satisfactory and are described here. The availability of three 

options will allow flexibility on the part of the party purchasing the test. Each of these options will 

produce results that can be used to determine the need for mitigation. 

Both Options 1 and 2 require the use of two measurements made for similar durations. Both 

measurements should report results in units of pCi/L or both in WL. Similar durations means that the 

two measurements must be made for a similar time period, with a two-hour grace period. Specific 

information on measurement methods (listed in Exhibit 3-1) can be found in EPA's Indoor Radon and 

Radon Decay Product Measurement Device Protocols (EPA 520-402-R-92-004; U.S. EPA 1992c). 

Exhibit 3-1

Radon and Radon Decay Product Measurement Method Categories

A 

(pCi/L)

B 

(WL)

AC Activated Charcoal Adsorption 

integrating RP Radon Progeny Sampling unit 

AT Alpha Track Detection CW Continuous Working Level 

Monitoring 

LS Charcoal Liquid Scintillation 

CR Continuous Radon Monitoring 

PB Pump-collapsible Bag 

SC Evacuated Scintillation Cell 

(three-day integrating) 

EL Electret ion Chamber: Long Term 

ES Electret ion Chamber: Short Term 

UT Unfiltered Track Detection 
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3.2 Options for Real Estate Testing 

3.2.1 Option 1: Sequential Testing

Sequential tests should be conducted under conditions that are as similar as possible, in the same 

location, and using similar devices and durations. Both should produce results in the same units (pCi/L 

or WL). That is, both methods should be from column A or both from column B of Exhibit 3-1. Any EPArecognized method may be used. In addition, the results of the first test should not be reported prior to 

making the second measurement; both measurements should be reported at the same time in order to 

discourage tampering that may occur if the first test is known to be greater than 4 pCi/L or 0.02 WL. 

Note that measuring with different methods (e.g., with AC and ES) may increase the potential for 

differences (e.g., measurement bias) between the results. The results of both measurements should be 

reported, and the average of the two results should be used to determine the need for mitigation. There 

will be some variation between the two results, which may be caused by the radon levels fluctuating in 

response to weather or other factors. If the variation is unusually large, it may be due to weather or 

other effects, but the measurement system should be checked for possible problems. 

3.2.2 Option 2: Simultaneous Testing

This option involves the use of two tests, conducted simultaneously and side-by-side, made for similar 

durations, and producing results in the same units (i.e., both methods should be from column A or both 

should be from column B of Exhibit 3-1). Any EPA-recognized method may be used. As with Option 1, 

using different methods for the two measurements (for example, ES and LS) may increase the potential 

for differences between the two results. The two test results should be averaged to determine the need 

for remedial action. The collocated devices should be placed four inches (10 centimeters) apart. 

Because radon measurements, like any measurements, usually do not produce exactly the same 

results, even for simultaneous testing, there will usually be a difference between the two results. EPA 

offers the following guidance to testers for judging when two simultaneous, side-by-side measurements 

disagree to such an extent that two additional measurements should be performed. 

The results of the simultaneous measurements will fall into one of the three categories discussed below 

and illustrated in Exhibit 3-2. 

3.2.2.1 Both Measurement Results Equal To or Greater Than 4 pCi/L (or 0.02 WL)

In this case, the average of the two results will be equal to or greater than 4 pCi/L, or 0.02 WL, and 

mitigation is recommended. The tester should report both measurement results as well as the average 

of the two results. 
- 16 - 

Exhibit 3-2

Deciding on a Retest When Measurements Vary Significantly

3.2.2.2 Both Measurement Results Less Than 4 pCi/L (or 0.02 WL)

In this case, the average of the two measurements will be less than 4 pCi/L, or 0.02 WL, and both 

measurement results and the average result should be reported to the client. 

3.2.2.3 One Measurement Result Greater Than 4 pCi/L (or 0.02 WL), and One Measurement Result 

Less Than 4 pCi/L (or 0.02 WL)

This is a special situation in which the average of the results is critical. To assist testers in ensuring that 

the difference between two measurements is small enough so that clients may have confidence in, and 

understand, the results, EPA offers the following simple guidance. 

If the higher result is twice or more the lower result, then the two results are not within a factor of two, 

and a retest should be conducted. Section 3.2.2.5 provides language for informing the client that a 

retest is warranted. 

If the higher result is less than twice the lower result, then the two results are within a factor of two, and 

a retest is not necessary. The results of both measurements and the average of the two results should 

be reported to the client. (See Section 4 for more detailed information on quality assurance and quality 

control procedures.) 

3.2.2.4 Precision Recommendations

Measurements near the lower limit of detection (LLD) for the measurement system often have large 

and varying precision errors, and it is difficult to assign any sort of probability level to very low results. 
- 17 - 

Simultaneous measurement results that are equal to 4 pCi/L, or 0.02 WL, or greater should, however, 

exhibit some agreement. An example control chart for the precision that may be expected is shown as 

Exhibit B-2 in Appendix B, which was constructed using an average relative percent difference of 14 

percent. (Relative percent difference is defined as the difference divided by the average.) Using Exhibit 

B-2, a relative percent difference greater than 36 percent should be observed less than one percent of 

the time. Based upon this, EPA recommends that any side-by-side, simultaneous measurements with 

results greater than or equal to 4 pCi/L, or 0.02 WL, and which exhibit a relative percent difference 

greater than 36 percent, be cause for informing the client that the two results do not show good 

agreement. However, since both results are greater than 4 pCi/L, or 0.02 WL, EPA recommends 

mitigation in this case. Testers should investigate the source of the error (see Appendix B). 

Results between 2 pCi/L (or 0.01 WL) and 4 pCi/L (or 0.02 WL), should also exhibit some agreement. 

The level of agreement expected should be based upon the tester's experience with duplicate 

measurements made with that technique in this range of radon concentrations. An example control 

chart for the precision that may be expected in this region is shown as Exhibit B-3 in Appendix B, which 

was constructed using an average relative percent difference of 25 percent. Using this chart, a relative 

percent difference between duplicates greater than 67 percent should be observed less than one 

percent of the time. Based upon this, EPA recommends that any side-by-side, simultaneous 

measurements with results less than 4 pCi/L, or 0.02 WL, and which exhibit a relative percent 

difference greater than 67 percent, be cause for informing the client that the two results do not show 

good agreement, but that both are less than 4 pCi/L, or 0.02 WL, and therefore mitigation is not 

recommended. Testers should investigate the source of the error (see Appendix B). 

3.2.2.5 Recommended Language for Informing the Client that a Retest is Warranted

If a retest is warranted (see Section 3.2.2.3), EPA recommends that the tester inform the client that 

EPA provides guidance for how well two measurements should agree, that the measurements 

performed fall outside the range, and that a retest should be conducted. A retest should consist of 

measurements performed according to one of the protocols outlined in Sections 3.2.1, 3.2.2, or 3.2.3.

3.2.3 Option 3: Single Test Option

This option requires an active continuous monitor (method CR or CW) that has the capability to 

integrate and record a new result at least hourly. Shorter integration periods and more frequent data 

logging afford greater ability to detect unusual variations in radon or radon decay product 

concentrations. The minimum measurement period is 48 hours. The first four hours of data from a 

continuous monitor may be discarded or incorporated into the result using system correction factors 

(EPA 520-402-R-92-004; EPA 1992c). There must be at least 44 contiguous hours of usable data to 

produce a valid average. (The "backing out" of data [i.e., removal of portions imbedded in the two days] 

to account for weather or other phenomena will invalidate the measurement.) The periodic results 

should be averaged to produce a result that is reported to the client. 

If the monitor cannot integrate over a period of one hour or less then an additional (secondary) passive 

or active measurement device must be used. The second measurement, which may be made with a 

passive or active device, can be used simultaneously or sequentially, as discussed in Option 1 and 2 

(Sections 3.2.1 and 3.2.2). If the two measurements are performed simultaneously, their results should 

be evaluated following the guidance in Section 3.2.2 If these two results will be different. As discussed 

in Section 3.2.1, the difference between sequential tests may be due to radon levels fluctuating in 

response to weather or other factors. 

In general, confidence in a radon measurement can be increased by performing another measurement 

with a second measurement device. However, there are other approaches or features that can be used 

to increase the confidence of a measurement results obtained using active monitor devices. These 
- 18 - 

approaches include the use of device self-diagnostic features and data validation or verification 

procedures, that could be employed before and/or after the measurement. Example of such 

approaches are the use of check sources before and after each measurement, and use of spectrum 

readouts. These capabilities are examples, and different technologies may be able to perform other 

similar self-diagnostic or quality assurance checks. Other features that increase the confidence of a 

single active test include (but are not limited to) the ability to check air flow rates and voltage meters 

before and after each measurement. Measurement companies should incorporate such checks into 

their routine instrument performance checks as part of their standard operating procedures. 

Additional features that can increase confidence in measurement results are those that detect 

measurement interference; these features are discussed in Section 3.5. For example, a devise that 

offers a variety of ways to detect tampering may serve to deter, as well as detect, interference with the 

device's operation or proper closed-building measurement conditions. Potential tampering indicators 

include the ability of a device to record changes in temperature, humidity, or movement of or around 

device during the measurement. 

Instruments with greater efficiency or sensitivity, or a high signal-to-noise ratio (see Glossary for 

definitions of these terms), can achieve results with a smaller uncertainty than instruments with low 

efficiency, poor sensitivity, or low signal-to-noise ratio. Greater efficiencies, sensitivities, or a hight 

signal-to-noise ratio may also facilitate tempering detection by being more sensitive to fluctuations in 

radon levels. There have been recommendations for setting minimum efficiency standards for active 

decices at 16 counts per hour per hour per pCi/L. EPA plans to conduct research to establish minimum 

standards in the future for all categories of devices, passive as well as active detectors. The reliability 

of any type of equipment, however, needs to be established and documented via a complete quality 

assurance program. This includes routine instrument performance checks prior to and after each 

measurement, annual calibrations, semi-annual instrument cross-checks, the performance of duplicate 

measurements in 10 percent of the measurement locations, and frequent background and spiked 

measurements. 

3.3 Measurement Location 

EPA recommends that measurements made for a real estate transaction be performed in the lowest 

level of the home which is currently suitable for occupancy. This means the lowest level that is 

currently lived-in, or a lower level that is not currently used (such as a basement, which a buyer could 

use for living space without renovations). Measurements should be made in a room that is used 

regularly, such as a living room, playroom, den, or bedroom. This includes a basement that can be 

used as a recreation room, bedroom, or playroom. This provides the buyer with the option of using a 

lower level of the home as part of the living area, with the knowledge that it has been tested for radon. 

3.4 Measurement Checklist 

EPA presents the following checklist to help ensure that a radon measurement conducted for a real 

estate transaction is done properly. The seller, or an EPA-listed or State-listed tester should be able to 

confirm that all the items in this checklist have been followed. If the tester cannot confirm this, another 

test should be made. 

Before the radon test: 

• Notify occupants of the importance of proper testing conditions. Give occupants written 

instructions or a copy of the EPA Home Buyer's and Seller's Guide to Radon (EPA 402-R-93-

003; U.S. EPA 1993), or a State-required alternative, and explain the directions carefully. 
- 19 - 

• The radon measurement service and device should be listed by EPA's National Radon 

Measurement Proficiency (RMP) program (EPA 520/1-91-006; U.S. EPA 1991) or listed by your 

State. Follow the manufacture's instructions that come with the device. 

• The radon measurement equipment used should be listed by some proficiency organization or 

listed by your State. Follow the manufacturer's instructions that come with the device. 

• If a testing professional conducts the test, only EPA-listed or State-listed individuals should be 

hired. Their photo identification should be provided to the client or homeowner at the time of, or 

before, the test, and the contactor's identification number should be clearly visible on the test 

report. 

• The test should include method(s) to prevent or detect interference with testing conditions or 

with the testing device itself. 

• Conduct the radon test for minimum of 48 hours. Some devices must be exposed for longer 

than the 48-hour minimum. 

• In homes with an active radon reduction system, check that the fan is running at least 24 hours 

before starting a short-term test lasting less than four days. Air exhaust fans that typically 

operate for short periods (e.g., bathroom fan) may be used during the test. 

• EPA recommends that short-term radon testing, which lasts for no more than a week in length, 

be done under closed-building conditions. Closed-building conditions means keeping all 

windows closed, keeping doors closed except for normal entry and exit, and not operating fans 

or other machines that bring in air from outside. Note that fans that are part of a radon reduction 

system or small exhaust fans operating for only short periods of time may run during the test. 

• When doing short-term testing lasting less than four days, it is important to maintain closedbuilding conditions for at least 12 hours before the beginning of the test and for the entire test 

period. Do not operate fans or other machines that bring in air from the outside. 

During the radon test: 

• Maintain closed-building conditions during the entire time of a short-term test, especially for 

tests shorter than one week in length. 

• Operate the home's heating and cooling systems normally during the test. For tests lasting less 

than one week, only operate air conditioning units that recirculate interior air. 

• Do not disturb the test device at any time during the test. 

• If a radon reduction system is in place, make sure the system is working properly and will be in 

operation during the entire radon test. 

After a radon test: 

• If a high radon level is confirmed, fix the home. Pages 21 to 23 of EPA's Home Buyer's and 

Seller's Guide to Radon (EPA 402-R-93-003; U.S. EPA 1993) recommends the next steps that 

should be taken, such as contacting a qualified radon reduction contractor to lower the home's 

radon level. 

• The radon tester or homeowner should be able to verify or provide documentation asserting that 

testing conditions were not violated during the testing period. 
- 20 - 

3.5 Interference-Resistant Testing 

EPA strongly encourages the use of radon testing devices with interference-resistant features inherent 

in, or associated with, the device. 

Interference with a radon measurement is defined as the altering of test conditions prior to or during the 

measurement to either change the radon or decay product concentrations or alter the performance of 

the measurement equipment. The following discussion reviews some of the types of test interferences 

and methods of detecting and preventing such interferences. 

Test interference typically causes measurement results to be different than if all proper test conditions 

were maintained. False low results have been primarily associated with testing during a real estate 

transaction, although they also happen when the occupants of the dwelling are not properly informed 

about the necessary test conditions. Test interference can also inadvertently increase measurement 

results, although the intent is to lower the results. 

The current occupant may have an interest in the test results being as low as possible to avoid 

hindering the sale of the dwelling or incurring the added expense of having to install a mitigation 

system. The potential for test interference puts the professional radon tester into the position of 

verifying that the equipment and the required test conditions have been maintained. A measurement 

result that is below the action guideline may be suspect if the tester cannot verify that the necessary 

test conditions were maintained. 

If the tester arrives at a property and finds windows or doors open, or suspects that closed-building 

conditions were not maintained for 12 hours prior to arrival, then the tester should extend the test to 

account for this condition. 

3.5.1 Influencing Test Area Concentration

The primary method of temporarily reducing radon levels is to ventilate the test area with outdoor air. 

Ventilation will slow down radon entry by both reducing negative pressure in the test area and by 

diluting the reduced radon concentration. Even small openings of a single window in the test area can 

have a large effect. Ventilating the floors above the test area has significantly less effect, unless the test 

area is connected with the ventilated room(s) by an operating central air handling system. 

Radon decay product levels are sensitive to air movement. As air movement increases, decay products 

will plate out on walls and other surfaces, including fans, thereby reducing airborne decay product 

concentrations. Decay products will be further reduced if the fan also includes a filter. Radon levels are, 

however, not affected by filtering or air movement. 

It is also possible to alter concentrations in a tight room if the heating system is operating in an 

abnormal fashion. Since this may not be the typical operation of the system, it is, in effect, interfering 

with normal house conditions. 

It is important to recognize that test interference can increase radon or decay product levels, despite 

intent to lower the results. 

3.5.2 Equipment Interference

The primary method of interfering with testing equipment is to move the detector to an area of low 

radon concentration. Other types of interference vary in their ability to influence different types of 

detectors. For example, interfering with the air sampling mechanisms can maintain the radon 

concentration at the time of interference, or cause a large decrease in the reported concentration. 
- 21 - 

Similarly, covering a decay product or charcoal detector could cause a large drop in the reported 

values, while other types of radon detectors would only show a reduced response time to changes in 

the test area level. In addition, charcoal detectors are sensitive to heat. Some active radon monitors 

and open face charcoal canisters are also sensitive to high humidity. Any detector that yields a single 

result could be turned off or sealed in its container or lid during most of its exposure period. 

3.5.3 Preventing Interference

EPA recommends that a radon measurement conducted for a real estate transaction be performed 

using tamper-resistant testing techniques. It is more advantageous for the tester to take steps to 

prevent interference rather than to simply detect it. Preventing interference can best be accomplished 

by: 

• Educating the parties to a real estates transaction about the necessary test conditions. 

• Including in standard documentation for each measurement an agreement signed by the parties 

involved in the real estate transaction listing the necessary test conditions and their agreement 

not to interfere with the conditions. 

The agreement should also state that the tester in their discretion may nullify the test results 

were rendered unreliable. 

• Informing the client that interference with the test conditions may increase the radon levels. 

• Informing the client that the tester is using interference-detecting techniques, and that these 

allow the detection and documentation of test interference. 

3.5.4 Interference-Resistant Detectors

The following is a partial list of common equipment and measures that can serve to prevent and/or 

detect test interference. There may be other methods available. Equipment that offers a combination of 

tamper-detecting features also offers a greater chance of detecting interference. 

• The ability to integrate and record frequent radon measurements over short intervals (an 

hour or less) is an important tamper detection feature. Continuous (active) monitors that 

provide frequent measurements can indicate unusual concentration changes that can be 

indicators of test interference. 

• Measuring other parameters may provide additional indicators of test interference, such as a 

detector tilt indicator or a continuous recording of pump flow rate. 

• A motion indicator can also indicate when the detector was approached or moved. 

• A simultaneous, several-day continuous measurement of both radon and decay product 

concentrations will produce a series of equilibrium ratio values. These values can be 

inspected for unusual swings or abnormal levels, possibly indicating interference. 

• Measurement of CO2 levels can indicate changes in the test area infiltration rate of outdoor 

air. 

• The performance of a grab radon measurement, a grab decay product measurement, or 

both, before and after a longer-term measurement can offer useful information. For 

example, the initial and final concentrations and equilibrium ratios can be compared for 
- 22 - 

consistency. Note: The results of measurements lasting less than 48 hours (e.g., grab 

samples) should not be used as the basis for deciding to fix a home. 

• Frequent temperature readings may help to indicate changes in the test area infiltration rate 

of outdoor air. 

• Humidity (as well as temperature) recordings can be especially helpful in identifying 

potential unusual changes in test conditions that occur during the test period that might not 

be detected simply through data logging. 

• Instruments that do not allow occupants to view preliminary results (via a visible printer or 

screen) may reduce occupants' interference. 

• Placement indicators can also indicate if a detector has been tampered with or moved. The 

position of the detector should be noted so that, upon retrieval, any handling of the detector 

can be indicated by a change in its position. A detector may be hung or placed slightly over 

the edge of its support to discourage covering it. Passive detectors may be hung or 

suspended in a radon-permeable bag that uses a unique strap and seal to prevent removing 

or covering it. Cages can be equipped with a movement indicator to deter handling of the 

cage or the detector within it. 

• Seals can aid in detecting and discouraging test interference, and they are especially 

important in the absence of other tamper detection. Non-sealable caulks and/or tapes can 

be used to verify that detectors have not been opened. Seals alone will not prevent 

excessive ventilation through primary doors. 

Seals should be placed on the lowest operable windows and non-primary exterior doors, as 

well as between the detector and its support and any other components of the detector that 

could be tampered with. It may be advisable to attach to the caulk seal something fragile 

that protrudes out, to indicate any handling or covering of the detector. 

A number of different products or combination of products can be used for tamper seals. For a seal to 

be effective, it needs at least the following unique qualities: 

 

• The seal must adhere readily to a multitude of surfaces, and yet be easily removed 

without marring the surface. 

• It needs to be non-resealable or show evidence of disturbance. 

• It must be unique enough to prevent easy duplication. 

• It should be visible enough to discourage tampering. 

The tamper resistance of the seal can be increased by using a caulk over the seal edges or by slicing a 

large portion of the center of the seal to ensure the seal is broken in case of tampering. Most paper or 

plastic tapes and caulks have only some of these qualities. There are, however, a number of seals 

manufactured specifically for radon testing. It would be advisable to use one of these products and 

follow the manufacturer's installation recommendations. The best caulking to use as a seal is a 

removable weatherstripping caulk. This type of caulking adheres readily to most surfaces, yet comes off 

easily without leaving a mark or being resealable. Upon retrieval of the detector, the tester should 

carefully inspect the following: 
- 23 - 

• That all closed-building conditions are still being maintained; 

• Any changes in the detector placement; 

• The condition of all seals; and 

• Any abnormal variations in any of the measurements made. 

This information should be recorded, as described in Section 4.3.5. 
- 24 - 

Section 4: General Procedural Recommendations 

4.1 Introduction

4.2 Initial Client Interview

4.3 Measurement Recommendations

4.4 Quality Assurance in Radon Testing

4.5 Standard Operating Procedures

4.6 Providing Information to Consumers

4.7 Reporting Test Results

4.8 Temporary Risk Reduction Measures

4.9 Recommendations for Mitigation

4.10 Worker Safety

4.1 Introduction 

This section outlines basic procedural recommendations for anyone involved in the measurement of 

radon in homes for both real estate and non-real estate related measurements. 

4.2 Initial Client Interview 

Reasonable efforts should be made to determine whether the home is new and/or occupied, and who 

will be in charge of the home during the measurement period. Testing organizations should inform the 

client and other parties to the real estate transaction of: 

• The appropriate EPA testing recommendations as outlined in this report, the 1992 Citizen's 

Guide to Radon (EPA 402-k-92-001; U.S. EPA 1992a) , or the Home Buyer's and Seller's Guide 

to Radon;(EPA 402-R-93-003; U.S. EPA 1993); and, 

• The types of devices they will be using for that test, and EPA documentation indicating that the 

testing organization or individual is RMP-listed for that device

4.3 Measurement Recommendations 

4.3.1 Selecting a Measurement Approach

The purpose of the measurements, as well as budget and time constraints, dictate the protocol used. 

Measurements made for the purpose of assessing the need for mitigation of one's own property should 

be made according to the guidance discussed in Section 2 of this document; Section 3 outlines options 

for protocols for measurements made for real estate transactions. Organizations that provide consultant 

services, or place or retrieve devices, should review the protocol options and the clients' needs, and 

inform clients of the buildings and test period conditions necessary for conducting valid measurements. 

In some areas, companies may offer different types of radon service agreements. Some agreements 

allow for a one-time fee that covers both testing, and if needed, radon reduction. 

The organization or individuals performing the measurement service should use only those specific 

devices or methods for which that organization or individuals is listed according to the National RMP 

Program (EPA 520/ 1-91-006; U.S. EPA 1991). Adherence to the EPA device protocols, Indoor Radon 

and Radon Decay Product Measurement Device Protocols (EPA 520-402-R-92-004; U.S. EPA 1992c) 

is a requirement for participation in the RMP program. 
- 25 - 

4.3.2 Written Measurement Guidance

Measurement organizations should provide clients with written measurement instructions that clearly 

explain the responsibilities of the client and the other parties to the real estate transactions during the 

test period. Written and verbal guidance should be in accordance with EPA's Indoor Radon and Radon 

Decay Product Measurement Device Protocols (EPA 520-402-R-92-004; U.S. EPA 1992c).and 

guidance published in the RMP Program Handbook (EPA 520/1-91-006; U.S. EPA 1991). 

• A statement of whether the device measures radon or radon decay products and a 

discussion of the units in which all results will be reported. 

The results of radon decay product measurements should be reported in working levels 

(WL). If the WL value is converted to a radon concentration and is reported to the 

homeowner, it should be stated that this approximate conversion is based on a 50 percent 

equilibrium ratio (unless the actual equilibrium ratio is determined). In addition, the report 

should indicate that this ratio is an assumed average found in the home environment; any 

indoor environment may have a different and varying relationships between radon and its 

decay products. 

• A description of closed-building conditions and a stated requirement that these conditions be 

maintained 12 hours prior to and during all short-term measurements lasting less than four 

days and preferably for those lasting up to one week. 

• Directions that the building's heating, ventilating, and air conditioning (HVAC) system and 

any existing mitigation system should be normally operated 24 hours prior to and during all 

measurements. 

• A permanent radon reduction system should be fully operational for at least 24 hours prior to 

testing to determine the mitigation system's effectiveness. The mitigation system is to be 

operated normally and continuously during the entire measurement 

• Specific information on the minimum and maximum duration of exposure for the device. 

• If the client will be performing the test, procedures for placing, retrieving, and handling the 

device. 

• A written non-interference agreement (see Sections 3.5.3 and 4.3.4) to be signed and 

returned by the real estate transaction which confirms that they followed all instructions and 

did not interfere with the conditions or the measurement device. 

4.3.3 Conditions for a Valid Measurement

Measurements should not be conducted if temporary radon reduction measures have been 

implemented. These included the introduction of unconditioned air into the home or closure or normally 

accessible areas of the home. In this case, the measurements organization or individual should inform 

the slient and other parties to the real estate transactions that these conditions have been corrected. 

A permanent radon reduction system should be fully operational for at least 24 hours prior to 

testing to determine the mitigation system's effectiveness. The mitigation system is to be operated 

normally and continuously during the entire measurements periods. 
- 26 - 

4.3.4 Non-Interference Controls

The measurement organization should provide parties to a real estate transaction clients with a written 

statement that discusses the importance of proper measurement conditions and of not interfering with 

the measurement device or building conditions. The reader should refer to Section 3.5.3 for more 

information on non-interference agreements. 

 Organizations that place and retrieve devices should, in addition to providing written guidance, take 

steps to identify attempts to interfere with the measurement device or building conditions. There is 

increasing use of non-interference agreements signed by parties involved in real estate transactions to 

help prevent interference with the radon test and test conditions. The reader should refer to Section 3.5

for more information on tamper-resistant testing. 

The signed non-interference agreement, a description of all non-interference controls employed, and a 

statement addressing any observed breaches of the non-interference agreement/controls should be 

made part of the permanent measurement documentation for each measurement. 

4.3.5 Measurement Documentation

Measurement organizations should record sufficient information on each measurement in a permanent 

log to allow for future data comparisons, interpretations, and reporting to clients. EPA recommends that 

a measurement log be kept with the following information and be maintained for five years. Additional 

method-specific documentation is outlined in EPA's Indoor Radon and Radon Decay Product 

Measurement Device Protocols (EPA 520-402-R-92-004; U.S. EPA 1992c). 

• A copy of the final report, including the measurement results, and the statement outlining 

any recommendations concerning retesting or mitigation provided to the building occupant 

or agent. 

• The address of the building measured, including zip code. 

• The exact locations of all measurement devices deployed. It is advisable to diagram the test 

area, noting the exact location of the detector. 

• Exact start and stop dates of the measurement period as required for analysis. 

• A description of the device used, including its RMP device identification number and serial 

number if any. 

• A description of the condition of any permanent vents, such as crawl space vents or 

combustion air supply to combustive appliances. 

• The name and PMP identification number (EPA 520/ 1-91-014-3N; U.S. EPA 1992e) of the 

service or analysis organizations used to analyze devices 

• The name and RMP identification number (or State license number) of the individual who 

conducted the test. 

• A description of any variations from or uncertainties about standard measurement 

procedures, closed-building conditions, or other factors that may affect the measurement 

result. 
- 27 - 

• A description of any non-interference controls used and copies of signed non-interference 

agreements. 

• A record of any quality control measures associated with the test, such as results of 

simultaneous or secondary measurements. 

4.4 Quality Assurance in Radon Testing 

Anyone providing measurement services using radon or radon decay product measurement devices 

should establish and maintain a quality assurance program. These programs should include written 

procedures for attaining quality assurance objectives and a system for recording and monitoring the 

results of the quality assurance measurements described below. EPA offers general guidance on 

preparing quality assurance plans (QAMS-005/80; U.S. EPA 1980); a draft standard prepared by a 

radon industry group is also available (AARST 1991). The quality assurance program should include 

the maintenance of control charts and related statistical data, as described by Goldin (Goldin 1984), by 

EPA (EPA 600/9-76-005; U.S. EPA 1984), and in Appendix B of this document. 

4.4.1 Calibration Measurements

Calibration measurements are measurements made in a known radon environment, such as a 

calibration chamber. Detectors requiring analysis, such as charcoal canisters, alpha track detectors, 

electret ion chambers, and radon progeny integrating samplers are exposed in a calibration chamber 

and then analyzed. Instruments providing immediate results, such as continuous working level and 

radon monitors, should be operated in a chamber to establish individual instrument calibration factors. 

Calibration measurements must be conducted to determine and verify the conversion factors used to 

derive the concentration results. These factors are determined normally for a range of concentrations 

and exposure times, and for a range of other exposure and/or analysis conditions pertinent to the 

particular device. Determination of these calibration factors is a necessary part of the laboratory 

analysis, and is the responsibility of the analysis laboratory. These calibration measurement 

procedures, including the frequency of tests and the number of devices to be tested, should be 

specified in the quality assurance program maintained by manufacturers and analysis laboratories. 

4.4.2 Known Exposure Measurements

Known exposure measurements or spiked samples consist of detectors that have been exposed to 

known concentrations in a radon calibration chamber. These detectors are labeled and submitted to the 

laboratory in the same manner as ordinary samples to preclude special processing. The results of 

these measurements are used to monitor the accuracy of the entire measurement system. Suppliers 

and analysis laboratories should provide for the blind introduction of spiked samples into their 

measurement processes and the monitoring of the results in their quality assurance programs. All 

organizations providing measurement services with passive devices should conduct spiked 

measurements at a rate of three per 100 measurements, with a minimum of three per year and a 

maximum required of six per month. Providers of measurements with active devices are required to 

recalibrate instruments at least once every 12 months and perform cross-checks with RPP-listed 

devices at least once every six months. Participation in EPA's National RMP Program will not satisfy 

the need for annual calibration, as this program is a performance test, not a calibration procedure 
- 28 - 

4.4.3 Background Measurements

Background measurements are required both for continuous monitors and for passive detectors 

requiring laboratory analysis. Users of continuous monitors must perform sufficient instrument 

background measurements to establish a reliable instrument background and to check on instrument 

operation. For more specific information on how often background measurements should be made, 

refer to EPA's Indoor Radon and Radon Decay Product Measurement Device Protocols (EPA 520-402-

R-004; U.S. EPA 1992c). 

Passive detectors requiring laboratory analysis require one type of background measurement made in 

the laboratory and another in the field. Suppliers and analysis laboratories should measure routinely the 

background of a statistically significant number of unexposed detectors from each batch or lot to 

establish the laboratory background for the batch and the entire measurement system. This laboratory 

blank value is subtracted routinely (by the laboratory) from the field sample results reported to the user, 

and should be made available to the users for quality assurance purposes. In addition to these 

background measurements, the organization performing the measurements should calculate the lower 

limit of detection (LLD) for its measurement system (Altshuler and Pasternack 1963, ANSI 1989, U.S. 

DOE 1990). This LLD is based on the detector and analysis system's background and can restrict the 

ability of some measurement systems to measure low concentrations. 

Providers of passive detectors should employ field controls (called blanks) equal to approximately five 

percent of the detectors that are deployed, or 25 each month, whichever is smaller. These controls 

should be set aside from each detector shipment, kept sealed and in a low radon environment, labeled 

in the same manner as the field samples to preclude special processing, and returned to the analysis 

laboratory along with each shipment. These field blanks measure the background exposure that may 

accumulate during shipment and storage, and the results should be monitored and recorded. The 

recommended action to be taken if the concentrations measured by one or more of the field blanks is 

significantly greater than the LLD is dependent upon the type of detector. More information is available 

in EPA's Indoor Radon and Radon Decay Product Measurement Device Protocols (EPA 520-402-R-92-

004; U.S. EPA 1992c). 

4.4.4 Duplicate Measurements

Duplicate measurements provide a check on the quality of the measurement result, and allow the user 

to make an estimate of the relative precision. Large precision errors may be caused by detector 

manufacture, and/or improper data transcription or handling by suppliers, laboratories, or technicians 

performing placements. Precision error can be an important component of the overall error, so it is 

important that all users monitor precision. 

Duplicate measurements for both active and passive detectors should be side-by-side measurements 

made in at least 10 percent of the total number of measurement locations, or 50 each month, whichever 

is smaller. The locations selected for duplication should be distributed systematically throughout the 

entire population of samples. Groups providing measurement services to homeowners can do this by 

providing two measurements, instead of one, to a random selection of purchasers, with the 

measurements made side-by-side. As with spiked samples introduced into the system as blind 

measurements, the precision of duplicate measurements should be monitored and recorded in the 

quality assurance records. The analysis of data from duplicates should follow the methodology 

described in Appendix B of this document. If the precision estimated by the user is not within the 

precision expected of the measurement method, the problem should be reported to the analysis 

laboratory and the cause investigated. 
- 29 - 

4.4.5 Routine Instrument Performance Checks

Proper functioning of analysis equipment and operator usage require that the equipment and 

measurement system be subject to routine checks. Regular monitoring of equipment and operators is 

vital to ensure consistently accurate results. Performance checks of analysis equipment includes the 

frequent use of an instrument check source. In addition, important components of the device (such as a 

pump and pump flow rate, battery, or electronics) should be checked prior to each measurement and 

the results noted in a log. Each user should develop methods for regularly (daily, or at least prior to 

each measurement) monitoring their measurement system, and for recording and reviewing results. 

4.4.6 Quality Assurance Plans

All organizations should develop, implement, revise periodically, and maintain a detailed quality 

assurance plan (QAP) appropriate to each device or method used. This was a requirement for 

participation in EPA's National Radon Measurement Proficiency (RMP) program. Specific guidance on 

the necessary quality control measures for each measurement method is provided in EPA's Indoor 

Radon and Radon Decay Product Measurement Device Protocols (EPA 520-402-R-92-004; U.S. EPA 

1992c). 

Organizations that do not use continuous monitors or do not analyze detectors also need to write and 

follow a QAP, and conduct quality control measurements. These include duplicate, blank, and spiked 

measurements as described in Section 4.4. For more information on EPA's RMP Program, please 

contact: 

RMP Program Information Services 

Research Triangle Institute 

3040 Cornwallis Road-Building 7 

P.O. Box 12194 

Research Triangle Park, NC 27709-2194 

(919-541-7131/FAX -7386) 

4.5 Standard Operating Procedures 

Organizations performing radon measurements should have a written, device-specific standard 

operating procedure (SOP) in place for each radon measurement system they use. An SOP must 

include specific information describing how to operate and/or analyze a particular measurement device. 

Organizations that analyze devices should develop their own SOP or adapt manufacturer-developed 

SOPs for their devices. Organizations that receive results from a laboratory should have a devicespecific SOP for each brand/model/type of device that they use. All SOPs should be consistent with the 

appropriate protocol outlined in EPA's Indoor Radon and Radon Decay Product Measurement Device 

Protocols (EPA 520-402-R-92-004; U.S. EPA 1992c). 

4.6 Providing Information to Consumers 

Organizations should provide the customer with the following information: 

• Devices that will be placed by the customer must be accompanied by instructions on how to use 

the device. These instructions should be consistent with EPA's Indoor Radon and Radon Decay 

Product Measurement Device Protocols (EPA 520-402-R-92-004; U.S. EPA 1992c) and include 

specific information on the minimum and maximum length of time that the device must be 

exposed. 
- 30 - 

• The service organization should inform clients about sources of information on mitigation, such 

as EPA's Consumer's Guide to Radon Reduction (EPA 402-K-92-003; U.S. EPA 1992b), and 

other information available through their State Radon office. The organizations should also 

provide any State-required brochures which provide information on mitigation. 

• If service organization distribute the Consumer's Guide brochure, it should be reproduced in its 

entirety. 

4.7 Reporting Test Results 

Organizations should return radon measurement results to clients within a few weeks of retrieving 

exposed devices or receiving an exposed device which has been delivered for analysis. At a minimum, 

the client report should contain the following information: 

• Measurement results reported in the units that the device measures. Any measurement results 

based on radon gas (pCi/L of air) should be reported to no more than one decimal place, e.g., 

4.3 pCi/L. Any measurement result based on radon decay products (WL) should be reported 

to no more than three decimal places, e.g., 0.033 WL. Any conversions from WL to pCi/L or 

from pCi/L to WL should be presented and explained clearly. 

If the WL value is converted to a radon concentration, it should be stated in the report to the 

homeowner that this approximate conversion is based on a 50 percent equilibrium ratio (unless 

the actual equilibrium ratio is determined). In addition, the report should indicate that this ratio is 

typical of the home environment, but that any indoor environment may have a different and 

varying relationship between radon and its decay products. 

• The dates of the measurement period and address of the building tested. 

• A description of the device used, its manufacturer, model or type, and the device identification 

(serial) numbers. 

• The name and any relevant identification numbers of the organization and individual placing and 

retrieving the device and the organization analyzing the device, if they are different. 

• A statement concerning any observed tampering or deviations from the required test conditions. 

• Organizations that offer measurement services with grab sampling devices should provide 

clients with written notification stating that grab sample results can be useful diagnostic tools, 

but should not be used for deciding whether or not to mitigate. 

• Diagnostic measurements should be reported as "for diagnostic purposes only." 

4.8 Temporary Risk Reduction Measures 

Contractors should refer home's occupants and real estate agents to EPA's Radon Mitigation 

Standards (U.S. EPA 1992d) or the Consumer's Guide to Radon Reduction (EPA 402-K-92-003; U.S. 

EPA 1992b) for information on temporary and permanent risk reduction measures. 

If any radon reduction efforts are identified during measurement procedures, testers should inform 

clients and other parties to the real estate transaction that altered conditions during the measurement 

will invalidate the results and decline to conduct a measurement until the conditions have been 

corrected. 
- 31 - 

4.9 Recommendations for Mitigation 

The measurement organization should inform consumers that EPA recommends fixing houses with 

radon levels equal to or greater than 4 pCi/L, and that EPA recommends in its "Consumer's Guide to 

Radon Reduction" the use of EPA Radon Contractor Proficiency (RCP)-listed and/or State-listed 

mitigation contractors to perform the work. 

Organizations should refer customers to their State radon office for copies of EPA's "Consumer's Guide 

to Radon Reduction" (EPA 402-K-92-003; U.S. EPA 1992b) and a list of EPA RCP-proficient and Statelisted mitigators. 

Homes should also be tested again after they are fixed to be sure that radon levels have been reduced. 

If the occupants' living patterns changes and they begin occupying a lower level of their home (such as 

a basement), the home should be retested on that level. In addition, it is a good idea for homes to be 

retested sometime in the future to be sure radon levels remain low. 

4.10 Worker Safety 

Individuals and organizations should comply with all applicable Occupational Safety and Health 

Administration (OSHA) standards and guidelines relating to occupational worker exposure, health, and 

safety. Information on worker health and safety contained in EPA or State publications is not 

considered a substitute for any provisions of the Occupational Safety and Health Act of 1970 or for any 

standards issued by OSHA. 
- 32 - 

Appendix A - State and EPA Regional Radon Offices 

A.1/A.2 State and EPA Region Radon Offices - www.epa.gov/iaq/whereyoulive.html 
- 33 - 

Appendix B - Interpretation of the Results of Side-by-Side Measurements 

B.1. Assessment of Precision

B.2. Example Control Charts for Precision

B.3. Interpretation of Precision Control Charts

B.1 Assessment of Precision 

Radon and working level measurements, like all measurements, usually do not produce exactly 

the same results, even for collocated measurements. It is therefore critical to understand, document, 

and monitor the variability, or precision, of the measurements. This knowledge and proper will allow 

you to characterize precision error to clients. Further more the continual minitoring of precision provides 

a check on every aspect of the measurement system. 

The objective of performing simultaneous or duplicate measurements is to assess the precision 

error of the measurement method, or how well two side-by-side measurements agree. This precision 

error is the "random" component of error (as opposed to the calibration error, which is systematic). The 

precision error, or the degree of disagreement between duplicates, can be composed of many factors. 

These include the error caused by the random nature of counting radioactive decay, slight differences 

between detector construction (for example, small differences in the amount of carbon in activated 

carbon detectors), and differences in handling of detectors (for example, differences in accuracy of the 

weighing process, and variations of analysis among detectors). 

There is a variety of ways to quantitatively assess the precision error based on duplicate 

measurements. It is first necessary to understand that precision is characterized by a distribution; that 

is, your side-by-side measurements will exhibit a range of differences. There is some chance that any 

level of disagreement will be encountered, due merely to the statistical fluctuations of counting 

radioactive decays. The probability of encountering a very large difference between duplicates is 

smaller than the chance of observing a small difference similar to those that are routinely observed. It is 

important to recognize that a few high precision errors do not necessarily mean that the measurement 

system is flawed. 

Ideally, the results of duplicates should be assessed in a way that allows for the determination of what 

level of chance is associated with a particular difference between duplicates. This will allow for the predetermination of limits for the allowable differences between duplicates before an investigation into the 

cause of the large differences is made. For example, the warning level, or the level of discrepancy 

between duplicates which triggers an investigation, may be set at a five percent probability. This level is 

a difference between duplicates that is so large that, when compared with previous precision errors, 

should only be observed five percent of the time. A control limit, where further measurements should 

cease until the problem is corrected, may be set at one percent probability. 

A control chart for duplicates is not as simple as a control chart used to monitor instrument 

performance, as for a check source. This is because the instrument's response to a check source 

should be fairly constant with time. Duplicates are performed at various radon concentrations, however, 

and the total difference between two measurements is expected to increase as radon levels increase. 

Use of statistics such as the relative percent difference (RPD; difference divided by the mean) or the 

coefficient of variation (COV; standard deviation divided by the mean) can be used in a control chart for 

duplicate measurements at radon concentrations where the expected precision error is fairly constant in 

proportion to the mean, e.g., at levels greater than around 4 pCi/L or 0.02 WL. At lower concentrations, 

for example, between 2 pCi/L (or 0.01 WL) and 4 pCi/L (or 0.02 WL), a control chart may be developed 

by plotting these same statistics; however, the proportion of the precision error to the mean will be 
- 34 - 

greater than that proportion at levels above 4 pCi/L or 0.02 WL. At concentrations less than about 2 

pCi/L, or 0.01 WL, the lower limit of detection may be approached, and the precision error may be so 

large as to render a control chart not useful. 

Example control charts, using three different statistics, are described in the following sections. 

B.2 Example Control Charts for Precision 

Before a control chart can be developed, it is necessary to know, from a history of making good quality 

measurements with the exact measurement system (detectors, analysis equipment, and procedures), 

the level of precision that is routinely encountered when the system is operating well or "in control." It is 

that "in control" precision error that forms the basis of the control chart, and upon which all the 

subsequent duplicate measurements will be judged. There are two ways of initially determining this "in 

control" level. The first, and preferable, way is to perform at least 20 duplicate pairs of measurements at 

each range of radon concentrations for which a control chart is to be prepared. For example, if you will 

only assess precision at concentrations greater than 4 pCi/L, or 0.02 WL, you will need at least 20 pairs 

of measurements at concentrations greater than 4 pCi/L, or 0.02 WL, to assess the "in control" level. 

The average precision error (RPD or COV) should be the "in control" level. 

The second way to initially set the "in control" precision error level is to use a level that has been used 

by others, and that is recognized by industry and EPA as a goal for precision, for example, a 10 percent 

COV (corresponding to a 14 percent RPD). After at least 20 pairs of measurements are plotted, it will 

become apparent whether the 10 percent COV (or 14 percent RPD) is appropriate for your system. If 

itis not, a new control chart (using the guidelines below) should be prepared so that the warning and 

control limits are set at the correct probability limits for your system. 

B.2.1 Sequential Control Chart Based on Coefficient of Variation

It can be shown (Iglewicz and Myers 1970, EPA 600/9-76-005; U.S. EPA 1984) that when the expected 

precision is a constant function of the mean, control limits can be expressed in terms of the COV 

(COV=S/Xm; where S is the variance or the square of the standard deviation, and Xm is the mean or 

average of the two measurements). One method for obtaining percentiles for the distribution of the 

COV is to apply a chi-squared (X2

) test: 

X2

n-1 ≈ B[(n-1)COVn

2

/(n+(n-1)COVn

2

)] (Equation 1) 

where: B = n[1 + (1/COV2

)]; 

COVn = the observed COV of the nth pair (the pair that is to be evaluated); and 

COV = the "in control" COV (e.g., 10 percent at levels greater than 4 pCi/L). 

For duplicates, where n=2, Equation 1 becomes 

X2

 = [2 + (2/COV2)][COVn

2

/(2 + COVn

2

)] (Equation 2) 

For a value of 0.10 for COV, it further reduces to 

X2

 = 202[COVn

2

/(2 + COVn

2

)] (Equation 3) 

Referring to a X2

 chart, you learn that the probability of exceeding a X2 of 3.84 is only five percent. 

Inserting this value of 3.84 for X2

 and solving for COVn, produces a COVn of 0.20. This level of 
- 35 - 

probability forms the warning level shown in Exhibit B-1. The control limit corresponds to a X2 of 6.63 

and a COVn of 0.26, where the probability of exceeding those values is only one percent. 

This sequential control chart should be used by plotting results from each pair on the y-axis, and noting 

the date and measurement numbers on the x-axis. 

B.2.2 Sequential Control Chart Based on Relative Percent Difference

The RPD (or percent difference) is another expression of precision error, and is given by 

RPD = [100Ix1-x2I]/[(x1+x2)/2] (Equation 4) 

For n=2, 

RPD = COV SQRT 2 (Equation 5) 

The control limits for RPD can be obtained simply by multiplying the control limits for COV by the 

square root of two, or 1.41. These limits are shown in Exhibit B-2. This sequential control chart for RPD 

should be used in the same way as the control chart for COV, that is, with the vertical scale in units of 

RPD and the horizontal scale in units of date and measurement numbers. 

A control chart using the statistic RPD based on an "in control" level of 25 percent RPD is shown in 

Exhibit B-3. The warning level and control limit are set at 50 percent and 67 percent, respectively. Use 

of these limits may be appropriate for measured radon concentrations less than 4 pCi/L. 

Exhibit B-1

Control Chart* for Coefficient of Variation (COV) 

Based on an "In Control" Level of 10% 

(For duplicates where average >4 pCi/L or 0.02 WL) 
- 36 - 

COV=standard deviation of two measurements divided by their average 

Example: Detector A=5 pCi/L, B=6 pCi/L, COV=13% 

If COV exceeds the control limit--cease measurements until the problem is identified and corrected. 

If COV exceeds the warning level--follow guidance in Section B.3 and see Exhibit B-5. 

*As calculated from guidance provided in "Quality Assurance Handbook for Air Pollution Measurement 

Systems: Volume I" (EPA 600/9-76-005; U.S. EPA 1984) 

Exhibit B-2

Control Chart* for Relative Percent Difference (RPD) 

Based on an "In Control" Level of 14% (=COV of 10%) 

(For duplicates where average >4 pCi/L or 0.02 WL) 

RPD=difference between two measurements divided by their average 

Example: Detector A=5 pCi/L, B=6 pCi/L, RPD=18% 

If RPD exceeds the control limit--cease measurements until the problem is identified and corrected. 

If RPD exceeds the warning level--follow guidance in Section B.3 and see Exhibit B-5. 

*As calculated from guidance provided in "Quality Assurance Handbook for Air Pollution Measurement 

Systems: Volume I" (EPA 600/9-76-005; U.S. EPA 1984) 
- 37 - 

Exhibit B-3

Control Chart* for Relative Percent Difference (RPD) 

Based on an "In Control" Level of 25% (=COV of 18%) 

(For duplicates where average <4 pCi/L or 0.02 WL) 

RPD=difference between two measurements divided by their average 

Example: Detector A=2 pCi/L, B=3 pCi/L, RPD=40% 

If RPD exceeds the control limit--cease measurements until the problem is identified and corrected. 

If RPD exceeds the warning level--follow guidance in Section B.3 and see Exhibit B-5. 

*As calculated from guidance provided in "Quality Assurance Handbook for Air Pollution Measurement 

Systems: Volume I" (EPA 600/9-76-005; U.S. EPA 1984) 

B.2.3. Range Control Chart

A range control chart (Goldin 1984) can be constructed to evaluate precision, using the statistics of the 

range (difference between two measurements) plotted against the average of the two measurements. 

The control limits are again based on the variability of the measurements, as decided upon from 

previous results or using an industry standard (e.g., 10 percent). 

In this type of control chart, the limits are expressed in terms of the mean range (Rm), where, for n=2, 

Rm = 1.128 s(x) (Equation 6) 

where s(x) is the standard deviation of a single measurement, which reflects counting and other 

precision errors. Goldin shows that the limits can be expressed as follows: 
- 38 - 

Control limit = 3.69 s(x) (Equation 7) 

Warning level = 2.53 s(x) (Equation 8) 

An example range control chart, using an assumed s(x) equal to 10 percent of the mean concentration, 

is shown in Exhibit B-4. The chart is used by plotting the range versus average concentration as 

duplicate measurements are analyzed. 

Exhibit B-4

Range Control Chart to Evaluate Precision 

(Limits Based on s(x)=0.1xm) 

If results exceed the control limit--cease measurements until the problem is identified and corrected. 

If results exceed the warning level--follow guidance in Section B.3 and see Exhibit B-5. 

B.3 Interpretation of Precision Control Charts 

The control chart should be examined carefully every time a new duplicate result is plotted. If a 

duplicate result falls outside the control limit, repeat the analyses if possible. If the repeated analyses 

also fall outside the control limit, stop making measurements and identify and correct the problem. 

If any measurements fall outside the warning level, use the table in Exhibit B-5. Refer to the row 

showing the number of duplicate results outside the warning level. If the total number of duplicate 

results accumulated in the control chart is contained in column A, investigate the cause of the high level 

of precision error but continue making measurements. If the total number of duplicate results on the 

chart is contained in column B, stop making measurements until the cause for the high precision error 

is found, and it is determined that subsequent measurements will not suffer the same high level of 

precision error. 

Note that the example control charts shown here are simplifications of actual conditions, because they 

are premised on the assumption that the precision error is a constant fraction of the mean 
- 39 - 

concentration. In fact, the total precision error may best be represented by a different function of the 

mean concentration, for example, the square root of the concentration. The most accurate control chart 

can be rendered by a range control chart using the measurement uncertainty expressed as the 

standard deviation, s(x), expected at the concentrations where measurements are made. If the 

precision error is not a constant fraction of the mean, the control limits will not appear as straight lines, 

but may exhibit changing slope. However, methods discussed here present a conservative way to 

monitor, record, and evaluate precision error and are very useful for comparing observed precision 

errors with an industry standard. 

Exhibit B-5

Criteria for Taking Action for Measurements Outside the Warning Level*

Total Number of Duplicates 

Number of 

Duplicate Results 

Outside the 

Warning Level 

Investigate, But 

Continue Operations 

A

Stop Operations 

Until Problem is Corrected 

B

2 8-19 2-7 

3 17-34 8-16 

4 29-51 17-28 

5 41-67 29-40 

6 54-84 41-53 

7 67-100 54-66 

*Modified from Goldin (Goldin 1984) and based upon cumulative 

probability tables of the binomial distribution. 
- 40 - 

Glossary 

Accuracy: The degree of agreement of a measurement (X) with an accepted reference or true value 

(T); usually expressed as the difference (or bias) between the two values (X – T), or the difference as a 

percentage of the reference or true value (100[X – T]/T), and sometimes expressed as a ratio (X/T). 

Active radon/radon decay product measurement device: A radon or radon decay product 

measurement system which uses a sampling device, detector, and analysis system integrated as a 

complete unit or as separate, but portable, components. Active devices include continuous radon 

monitors, continuous working level monitors, and grab radon gas and grab working level measurement 

systems, but does not include devices such as electret ion chamber devices, activated carbon or other 

adsorbent systems, or alpha track devices. 

Alpha particle: Two neutrons and two protons bound as a single particle that is emitted from the 

nucleus of certain radioactive isotopes in the process of decay. 

Background instrument (analysis system, or laboratory) count rate: The nuclear counting rate 

obtained on a given instrument with a background counting sample. Typical instrument background 

measurements are: 

• Unexposed carbon: for activated carbon measurement systems. 

• Scintillaton vial containing scintillant and sample known to contain no radioactivity: for 

scintillation counters. 

• Background measurements made with continuous radon monitors exposed only to radon-free 

air (aged air or nitrogen). 

Background fields measurements (blanks): Measurements made by analyzing unexposed (closed) 

detectors that accompanied exposed detectors to the field. The purpose of field background 

measurements is to assess any exposure to the detector caused by radon exposure other than from 

the concentration in the environment to be measured. Results of background field measurements are 

subtracted from the actual field measurements before calculating the reported concentration. 

Background levels may be due to electronic noise of the analysis system, leakage of radon into the 

detector, detector response to gamma radiation, or other causes. 

Background radiation: Radiation arising form radioactivity material, the sun, and parts of the universe, 

other than that under consideration. Background radiation may also be due to the presence of 

radioactive substances in building materials. 

Becquerel (Bq): The International System of Units (SI) definition of Activity. 1 Bq = 1 disintegration per 

second. 

Calibrate: To determine the response or reading of an instrument relative to a series of known values 

over the range of the instrument; results are used to develop correction or calibration factors. 

Check source: A radioactive source, not necessarily calibrated, which is used to confirm the continuing 

consistent and satisfactory operation of an instrument. 

Client: The individual or parties who hire(s) the radon tester. 

Closed House Conditions: During any short-term test, closed-house-conditions should be maintained 

as much as possible while the test is in progress. In tests of less than 4 days duration, closed-houseconditions should be maintained for at least 12-hours before starting the test and for the duration of the 
- 41 - 

test. While closed-house-conditions are not required before the start of tests that are between 4 and 90-

days, closed-house-conditions should be maintained as much as possible. 

Coefficient of variation (COV), relative standard deviation (RSD): A measure of precision, 

calculated as the standard deviation (s or s) of a set of values divided by the average (Xave or μ), and 

usually multiplied by 100 to be expressed as a percentage. 

COV = RSD = (s/ Xave) x 100 for a sample, 

COV ' = RSD ' = (σ/μ) x 100 for a population 

See Relative percent difference. 

Curie (Ci): A commonly used measurements unit for radioactivity, specifically the rate of decay for a 

gram of radium – 37 billion decays per second. A unit of radioactivity equal to 3.7 x 1010 disintegrations 

per second. 

Duplicate measurements: Two measurements made concurrently and in the same location, or sideby-side. Use to evaluate the precision of the measurement method. 

Efficiency, Intrinsic detector: The relationship between the number of events recorded (counts, 

voltage lost, tracks) and the number of radioactive particles incident upon the sensitive element of the 

detector per unit time. Efficiencies for radon detectors are commonly expressed in terms of the 

calibration factor, which is the number of events (counts) per time (hour or minute) per radon 

concentration (pCi/L). Methods with high efficiencies will exhibit more counts (signal) per time in 

response to a given radon level than will a method with a low efficiency. 

Equilibrium ration, radon: Equilibrium ratio = [WL(100)]/(pCi/L). At complete equilibrium (i.e., at an 

equilibrium ratio of 1.0), 1 WL of RDPs would be present when the radon concentration was 100 pCi/L. 

The ratio is never 1.0 in a house. Due to ventilation and plate-out, the RPDs never reach equilibrium in 

a residential environment. A commonly assumed equilibrium ratio is 0.5 (i.e., the decay products are 

halfway toward equilibrium), in which case 1 WL would correspond to 200 pCi/L. However, equilibrium 

ratios vary with time and location, and ratios of 0.3 to 0.7 are commonly observed. 

Equilibrium equivalent concentration (EEC): The radon concentration in equilibrium with its shortlived progeny, that has the same potential alpha energy per volume as exists in the environment being 

measured (see working level). 

Exposure time: The length of time a specific mail-in device must be in contact with radon or radon 

decay products to get an accurate radon measurement. Also called exposure period, exposure 

parameters, or duration of exposure. 

Gamma radiation: Short-wavelength electromagnetic radiation of nuclear origin, with a wide range of 

energies. 

Integrating device: A device that measures a single average concentration value over a period of 

time. Also called a time integrating device. 

Lower limit of detection (LLD): The smallest amount of sample activity which will yield a net count for 

which there is confidence at a predetermined level that activity is present. For a five percent probability 

of concluding falsely that activity is present, the LLD is approximately equal to 4.65 times the standard 

deviation of the background counts (assuming large numbers of counts where Gaussian statistics can 

be used [ANSI 1989, Pasternack and Harley 1971, U.S. DOE 1990]). 
- 42 - 

Lowest Level suitable for occupancy: The lowest level currently lived in or a lower level not currently 

used, such as a basement, which a prospective buyer could use for living space without renovations. 

This includes a basement that could be used regularly, as for example a recreation room, bedroom, 

den, or playroom. 

Lowest lived-in level: The lowest level or floor of a home that is used regularly, including areas such 

as family rooms, living rooms, dens, playrooms, and bedrooms. 

Passive radon measurement device: A radon measurement system in which the sampling device, 

detector, and measurement system do not function as a complete, integrated unit. Passive devices 

include electret ion chamber devices, activated carbon or other adsorbent systems, or alpha track 

devices, but do not include continuous radon/radon decay product monitors, or grab radon/ radon 

decay product measurement system 

PicoCurie (pCi): One pCi is one trillionth (10^-12) of a Curie, 0.037 disintegrations per second, or 2.22 

disintegrations per minute. 

PicoCurie per liter (pCi/L): A unit of radioactivity corresponding to an average of one decay every 27 

seconds in a volume of one liter, or 0.037 decays per second a liter of air or water. 1 pCi/L = 37 

Becquerels per cubic meter (Bq/m3). 

Precision: A measure of mutual agreement among individual measurement made under similar 

conditions. Can be expressed in terms of the variance, pooled estimate of variance, range standard 

deviation at a particular concentration, relative percent difference, coefficient of variation or other 

statistic. 

Quality assurance: A complete program designed to produce results which are valid, scientifically 

defensible, and of known precision, bias, and accuracy. Includes planning, documentation, and quality 

control activities. 

Quality control: The system of activities to ensure a quality product, including measurements made to 

ensure and monitor data quality. Includes calibrations, duplicate, blank, and spiked measurements, 

inter-laboratory comparisons, and audits. 

Radon (Rn): A colorless, odorless, naturally occurring, radioactive, inert, gaseous element formed by 

radioactive decay of radium (Ra) atoms. The atomic number is 86. Although other isotopes of radon 

occur in nature, radon in indoor air is primarily Rn-222. 

Radon chamber: An airtight enclosure in which operators can induce and control different levels of 

radon gas and radon decay products. Volume is such that samples can be taken without affecting the 

levels of either radon or its decay products within the chamber. 

Relative percent difference (RPD): A measure of precision, calculated by: 

RPD = [(|X1 - X2 |)/ Xave] x 100 

where: 

X1 = concentration observed with the first detector or equipment; 

X2 = concentration observed with the second detector, equipment, or absolute value; 

|X1 - X2| = absolute value of the difference between X1 and X2; and 
- 43 - 

Xave = average concentration = ((X1 + X2) / 2) 

The relative percent difference (RPD) and coefficient of variation (COV) provide a measure of precision, 

but they are not equal. Below are example duplicate radon results and the corresponding values of 

relative percent difference and coefficient of variation: 

Rn1 

(pCi/L)

Rn2 

(pCi/L)

RPD 

(%)

COV 

(%)

8 9 12 8 

13 15 14 10 

17 20 16 11 

26 30 14 10 

7.5 10 29 20 

Note that the RPD divided by the square root of 2 = COV 

See Coefficient of variation (COV). 

Relative standard deviation: See Coefficient of variation. 

Sensitivity: The ability of a radon or WL measurement method to produce reliable measurements at 

low concentrations. This ability is dependent upon the variability of the background signal (counts not 

due to radon or WL exposure) which the method records, as well as its efficiency. Methods with stable 

background rates and high efficiencies will be able to produce reliable measurements at lower 

concentrations than methods with variable background rates and low efficiencies. Sensitivity can be 

expressed in terms of the lower limit of detection or minimum detectable activity. 

Signal-to-noise ratio: For radon and WL detectors, this term expresses the proportion of the number 

of counts due to exposure to radon or WP (signal) to the number of counts due to background (noise). 

Measurement methods with high signal-to-noise ratios will produce more counts due to radon or WL 

exposure (signal) in proportion to the background counts (noise) than will methods with low signal-tonoise ratios. A method with a high signal-to-noise ratio is more likely to exhibit good sensitivity, i.e., be 

able to produce reliable measurements at low concentrations. 

Spiked measurements, or known exposure measurements: Quality control measurements in which 

the detector or instrument is exposed to a know concentration in a calibration facility and submitted for 

analysis. Used to evaluate accuracy. 

Standard deviation (s): A measure of the scatter of several sample values around their average. For a 

sample, the standard deviation (s) is the positive square root of the sample variance: 

For a finite population, the standard deviation (σ) is: 
- 44 - 

where μ is the true arithmetic mean of the population and N is the number of values in the population. 

The property of the standard deviation that makes it most practically meaningful is that it is in the same 

units as the observed variable X. For example, the upper 95% probability limit on differences between 

two values is 2.77 times the sample standard deviation. 

Standard operating procedure: A written document which details an operation, analysis, or action 

whose mechanisms are prescribed thoroughly and which is commonly accepted as the method for 

performing certain routine or repetitive tasks. 

Statistical control chart, (Shewhart control chart): A graphical chart with statistical control limits and 

plotted values (for some applications in chronological order) of some measured parameter for a series 

of samples. Use of the charts provides a visual display of the pattern of the data, enabling the early 

detection of time trends and shifts in level. For maximum usefulness in control, such charts should be 

plotted in a timely manner (i.e., as soon as the data are available). 

Statistical control chart limits: The limits on control charts that have been derived by statistical 

analysis and are used as criteria for action, or for judging whether a set of data does or does not 

indicate lack of control. On a means control chart, the warning level may be two standard deviations 

above and below the mean, and the control limit may be three standard deviations above and below the 

mean. 

Systeme Internationale (SI): The International System of Units as defined by the Conference of 

Weights and Measures in 1960. 

Test Interference: The altering of test conditions prior to or during the measurement in order to change 

the radon or radon decay product concentrations or the altering of the performance of the measurement 

equipment. 

Time Integrated measurement: A measurement conducted over a specific time period (e.g., from two 

days to a year or more) producing results representative of the average value for that period. 

Uncertainty: The estimated bounds of the deviation from the mean value, expressed generally as a 

percentage of the mean value. Taken ordinarily as the sum of (1) the random errors (errors of 

precision) at the 95% confidence level, and (2) the estimated upper bound of the systematic error 

(errors of accuracy). 

Working level (WL): Any combination of short-lived radon decay products in one liter of air that will 

result in the ultimate emission of 1.3 x 105

 MeV of potential alpha energy. This number was chosen 

because it is approximately the alpha energy released from the decay products in equilibrium with 100 

pCi of Ra-222. Exposures are measured in working level months (WLM). 

Working level months (WLM): (working level x hours or exposure)/(170 hours/working month). In SI 

units, 1 WLM = 6 x 105

 Bq-h/M3 (EEC). 
- 45 - 

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