United States National Risk Management

Environmental Protection Research Laboratory

Agency Cincinnati, OH 45268

EPA Project Summary

Research and Development EPA/600/SR-96/022 March 1996

The Florida Radon Research Program

(FRRP), sponsored by the Environmental Protection Agency and the Florida

Department of Community Affairs, is

developing the technical basis for a

radon-control construction standard.

Results of the research conducted under the FRRP are presented in several

technical reports. This report is a summary of the technical basis for a candidate building materials radium standard. The report contains the standard

and a summary of the technical basis

for the standard.

This Project Summary was developed

by the National Risk Management Research Laboratory’s Air Pollution Prevention and Control Division, Research

Triangle Park, NC, to announce key

findings of the research project that is

fully documented in a separate report

of the same title (see Project Report

ordering information at back).

Introduction

Residential building materials, principally

concrete, can contain small amounts of

radium, which generates radon gas that

can migrate into residences. Furthermore,

radon daughters trapped in concrete emit

gamma radiation that can cause exposures to building occupants. This portion

of the standard defines acceptable limits

for the concentration of radium in building

materials.

Candidate Building Materials

Radium Standard

The candidate building materials radium

standard is:

Technical Basis for a Candidate

Building Materials Radium

Standard

Vern C. Rogers and Kirk K. Nielson

No material used in concrete for the

construction of habitable structures shall

have a total radium concentration that exceeds 10 pCi/g, as measured in accordance with procedures contained in “Standard Measurement Protocols, Florida Radon Research Program.” The radium concentration in concrete used in the construction of habitable structures shall not

have a total radium concentration that exceeds 5 pCi/g.

Radiation exposures from building materials to people can occur either from

indoor radon or from external gamma radiation. Both the concentration of indoor

radon and the level of gamma radiation

from the building materials depend primarily on the total radium concentration in

the materials. Thus, limiting the radium

concentration in the building material limits the amount of potential exposure to

building occupants from both indoor radon and gamma radiation. For typical concrete thicknesses (tens of centimeters),

the gamma radiation exposures yield about

the same radium concentration limits in

building materials as do exposures to indoor radon from the materials.

Technical Analysis Supporting

the Candidate Standard

The technical analysis supporting the

building materials standard consists three

steps:

1. Determine building material performance objectives for potential radon

and gamma exposures.

2. Calculate radium limit for indoor radon exposures.
2

3. Calculate radium limit for external

gamma radiation exposures.

Performance objectives were selected

according to their consistency with other

Florida Radon Research Programs. Specifically, the performance objectives are a

selected fraction of the U.S. Environmental Protection Agency’s (EPA) indoor radon (as radon-222) concentration guideline of 4 pCi/L and the EPA’s external

gamma radiation criterion of 20 μR/hr for

cleanup of off-pile lands associated with

uranium milling.

Indoor radon and gamma exposures to

occupants of residences can come from

many sources in addition to the building

materials. Soils under building structures

constitute the major source. Accordingly,

a reasonably conservative value of 25%

has been selected as the fraction of the

radon guidance and gamma radiation criterion attributed to building materials. Thus,

the building material performance objectives are:

1. Indoor radon-222 concentration limit -

1 pCi/L

2. External gamma radiation limit - 5 μR/

hr.

The calculations of indoor radon

concentration from a concrete slab

use an established procedure and

equation. Rearranging the

equation gives

Ra =

F

ρE λD tanh ( ) λ/D Xc

(1)

where:

Ra = radium-226 concentration limit in

the concrete (pCi/g)

F = radon flux (pCi/cm2 s)

ρ = concrete density (g/cm3)

E = radon emanation coefficient (unitless)

λ = radon decay constant (2.06x10-6/s)

D = radon diffusion coefficient of concrete (cm2

/s)

Xc= thickness of concrete slab (cm).

A simple approximation for F is available:

F = Chλ√(3.6x106) (2)

where:

C = indoor radon concentration

(pCi/L)

h = average indoor height of

building (cm)

λv= dwelling ventilation rate (air

changes/hour)

3.6x106 = units conversion (cm3 s/L hr)

Combining Equations (1) and (2) gives:

Ra =

Chλv

3.6x106

( ρE λD Xc ) (3)

The following values were used in the

calculations:

C = 1 pCi/L (performance objective)

h = 240 cm

λv= 0.25 air changes/hour

ρ = 2.1 g/cm3

E = 0.11

Xc= 10 cm

D = 2x10-3 cm2/s

Substituting these values into Equation

(3) gives a radium limit of 4 pCi/g in the

concrete.

This is rounded to 5 pCi/g in the Standard.

The radium concentration limit in concrete readily converts to radium concentration limits in concrete constituents. Since

aggregate comprises about 49% of Florida

residential concrete, the concrete aggregate radium limit can be estimated as

4 pCi/g/0.49, or 8 pCi/g in the aggregate.

A similar limit for sand is 10 pCi/g.

Exposures from gamma-emitting radionuclides in the concrete were estimated

by a slab with the same characteristics as

given in the full report. A 6x6-m area was

assigned to the slab, and the gammaradiation exposures were calculated with

the MICROSHIELD computer code.

The MICROSHIELD calculations gave

a value of 0.95 μR/hr per pCi/g of total

radium in the concrete. Thus, a radium

limit of about 5 pCi/g for the concrete gives

a potential exposure of 5 μR/hr. The corresponding total radium limit in the aggregate is 10 pCi/g, and that for the sand is

approximately 13 pCi/g.

Conclusions from Technical

Analysis

Potential exposures from building materials to occupants of dwellings occur from

external gamma radiation and from indoor

radon. The extent of these potential exposures is mainly determined by the radium

concentration in the materials. Both indoor radon exposure limits and external

gamma exposure limits yield a 5 pCi/g

radium concentration limit in concrete and

a 10 pCi/g radium limit in the aggregate

for the concrete.
3

V. Rogers and K. Nielson are with Rogers and Associates Engineering Corp., Salt

Lake City, UT 84110.

David C. Sanchez is the EPA Project Officer (see below).

The complete report, entitled “Technical Basis for a Candidate Building Materials

Radium Standard,” (Order No. PB96-157565; Cost: $17.50, subject to change)

will be available only from:

National Technical Information Service

5285 Port Royal Road

Springfield, VA 22161

Telephone: 703-487-4650

The EPA Project Officer can be contacted at:

Air Pollution Prevention and Control Division

National Risk Management Research Laboratory

U.S. Environmental Protection Agency

Research Triangle Park, NC 27711

• •

United States

Environmental Protection Agency

National Risk Management Research Laboratory (G-72)

Cincinnati, OH 45268

EPA/600/SR-96/022

Official Business

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