U.S. CONSUMER PRODUCT SAFETY COMMISSION

4330 EAST WEST HIGHW AY

BETHESDA, MD 20814

January 14, 2014

Ms. Diana Pappas Jordan

Chair of STP 2201

Underwriters Laboratories Inc.

333 Pfingsten Road

Northbrook, IL 60062

RE: CPSC Staff Request for Formation of a Working Group and Staff’s Recommendations for 

Requirements to Address the Carbon Monoxide Poisoning Hazard Associated with Portable 

Generators

Dear Ms. Jordan:

U.S. Consumer Product Safety Commission (CPSC, Commission) staff requests that Underwriters 

Laboratories Inc. (UL) form a working group to develop specific proposals for requirements for 

portable engine-generator sets that fall under the scope of UL 2201, Portable Engine-Generator 

Assemblies to reduce the risk of death and injury due to carbon monoxide (CO) poisoning.

a

 Since first 

joining the Standards Technical Panel (STP) for UL 2201 in 2002, CPSC staff has advocated that the 

STP adopt requirements in the standard to address the rising number of generator-related CO 

poisoning fatalities. Over the last few years, as part of the agency’s rulemaking project to address this 

hazard,

b CPSC staff has worked to demonstrate the feasibility of lowering engine CO emission rates. 

The results of this technical work conducted by and for the CPSC provide the solid foundation for

staff’s rationale and recommendations summarized herein for a framework of performance 

requirements and corresponding test methods that a working group can use as a starting point to 

develop specific proposals for consideration by the STP. 

Modifying UL 2201 to include CO emission rate limits would address the source of the hazard and 

make a direct impact on reducing the risk of CO poisoning death and injury associated with the use of 

portable generators.

c

 Despite sustained information and education efforts by the CPSC and many 

other stakeholders to promote public awareness of the generator-related CO hazard, especially before 

 a These comments are those of CPSC staff, and they have not been reviewed or approved by, and may not necessarily reflect

the views of, the Commission. 

b Portable Generators; Advance Notice of Proposed Rulemaking; Request for Comments and Information, Federal Register, 

71 FR 74472, December 12, 2006. CPSC staff is in the process of developing a draft notice of proposed rulemaking for the 

Commission’s consideration.

c Examples of reducing engine exhaust CO emission rates as the means to reduce CO deaths and injuries include forklifts and 

other equipment used in enclosed areas as well as the engines that power marine generators. Furthermore, when catalytic 

converters were put on automobile engines beginning in 1975 to meet EPA emission standards, it resulted in a reduction of 

unintentional vehicle-related CO deaths of greater than 80% in the years of 1975 through 1996 compared to earlier years.1

 

(Superscripted numbers refer to references listed at the end of this letter.)
2

and after major storms, the death toll continues to rise. The increase in fatalities has continued even 

since 2007, when CPSC mandated a hazard label.

2 According to the CPSC’s data, for the 14-year 

period from 1999 through 2012, there have been at least 800 consumer generator-related CO poisoning 

fatalities.

d,3

 In addition, in CPSC staff’s most recent report on non-fire CO deaths associated with use 

of consumer products, which provides annual estimates through 2009, generators have overtaken the 

entire product category of heating systems (including furnaces, portable heaters, and space heaters) to 

become the consumer product responsible for the largest estimated number of annual non-fire-related 

CO deaths since 2005.4

CPSC staff recommends a single performance requirement and corresponding test method that sets

a limit on the engine’s exhaust CO emission rate to a level that is technically achievable and will

modify the consequent CO exposure profiles to delay progression of CO poisoning symptoms 

compared to existing generators when operated in an enclosed space.e This will give exposed 

occupants more time to recognize that a hazardous situation is developing. This, in turn, will allow 

occupants greater opportunity to leave the exposure or to take actions that could result in their rescue

before becoming incapacitated. To assist in the development of a basis for this requirement, CPSC

engaged the National Institute of Standards and Technology (NIST) to perform tests on an unmodified,

carbureted 5 kilowatt (kW) generator operating in the attached garage of a single-family home to 

measure the accumulation of CO in the garage and transport throughout the house. Using the closed 

garage test data (a common fatal scenario), CPSC staff performed health effects modeling on the 

ensuing CO exposures, which predicts that occupants in the house and garage would have little time to 

perceive CO poisoning symptoms before the extremely quick onset of incapacitation and ultimately 

death.

5 Consequently, for this requirement, CPSC staff is currently considering an exhaust CO 

emission rate limit in the range of 100 to 150 grams per hour (g/hr) as the oxygen (O2) level in the 

intake air drops below ambient (20.9%). Oxygen depletion occurs when a generator is operated in an 

enclosed space and this can be an important factor in engine CO production. 

Staff’s analysis to determine the appropriate limit to recommend is ongoing; however, staff notes 

that staff’s prototype reduced-CO emission 5 kW portable generator emitted CO nominally at, or 

below 100 g/hr, and no trend toward higher emission rates was seen as the oxygen level dropped.5,7

This reduced CO emission rate was accomplished by retrofitting a closed-loop electronic fuel injection

system, an O2 sensor, and a catalyst onto the carbureted 5 kW generator. In contrast, NIST’s tests 

showed that the carbureted unit emitted CO at rates from a low of nominally 500 g/hr near ambient O2,

to a high of nearly 4,000 g/hr as O2 decreased to 17 percent. Using data from when NIST tested the 

prototype in the same attached closed garage scenario, CPSC staff’s health effects modeling predicts

that the garage occupant would experience a twelve-fold delay in progression of CO poisoning 

symptoms compared to the carbureted unit (96 minutes instead of 8 minutes for the time interval 

between predicted obvious symptom onset and incapacitation).

While CPSC staff is working with NIST in performing analyses to refine the details of possible 

requirements, staff offers the following general description of recommendations for a test procedure to 

determine the generator CO emission rate as the intake O2 level drops:

After situating the generator in a test chamber with air in the chamber initially set at 

approximately 68o

F, 40% relative humidity, and ambient CO concentrations, start the generator 

 d This count is based on 597 incidents entered into CPSC’s databases as of April 23, 2013.

e When the incident location was identified in CPSC’s investigation report of an incident, more than 95 percent of the 

incidents occurred when the generator was operated in an enclosed space.
3

and apply a continuous resistive load to one of the generator’s receptacles. Operate the generator

for at least 30 minutes and continue until either the chamber O2 drops to 16%, or the chamber O2

reaches steady state above 16% while the generator’s delivered power decreases to no more than

60% of the output measured at the beginning of the test. While the generator is operating, record

the chamber CO and O2 concentrations and the generator output power at least once per minute. 

Using the CO concentration data, the chamber’s air change rate, and the chamber volume, 

calculate the CO emission rate for each 5 minute interval with the load applied. Perform this test 

with each of the following three loads applied: the maximum load the generator will sustain 

without tripping its circuit breaker (this is defined as full load), 50% of full load, and 25% of full

load. To successfully meet the performance requirement, the CO emission rates at all three loads,

calculated as the O2 drops, must not exceed the emission limit, for which staff is currently 

considering a value in the range of 100 to 150 g/hr. The chamber volume and its range of 

achievable air change rates should be sized by the manufacturer according to the engine’s O2

consumption rates while operating under 25% load and full load for the size range of generators 

they expect to test.

It is possible that an exception to this performance requirement could be made to accommodate

alternative approaches suggested in public comments on the staff’s technical report on the prototype 

generator.6 Some commenters opined that a shutoff system, particularly one using a CO sensor, should 

be used as the means to reduce deaths and injuries (i.e., one that would shut off the engine before it

creates an unsafe CO exposure).

f It is important to note that although staff investigated four different

approaches for a shutoff system, including two that used CO sensors, staff was not able to demonstrate 

fully how a shutoff system could be implemented satisfactorily.

Accordingly, the working group should consider whether a generator equipped with an automated 

engine shutoff may be permitted to exceed the CO emission rate limit that staff is currently 

considering (in the range of 100 to 150 g/hr) when the O2 is below ambient O2 if all of the following 

three conditions are met:

1. The exhaust CO emission rate does not exceed the rate limit of 300 g/hr at ambient O2 (20.9%)

when full, 50%, and 25% loads, as defined above, are applied to the generator. This is 

expected to address the outdoor use scenario where not all of the exhaust from a generator 

operating outdoors at normal ambient O2 enters the building to create the occupants’ CO 

exposure profiles. This requirement is intended to provide those consumers who explicitly try 

to follow manufacturers’ instructions with the same increased time interval to recognize a 

hazardous situation is developing and, correspondingly, the same increased opportunity for 

escape as those who use the generator in an enclosed space.

g

 Staff’s analysis of the appropriate 

 f It is worth noting that the approach of a CO-sensing shutoff system was considered for a while by some stakeholders as a 

possible means to address CO deaths and injuries that were occurring on and around recreational boats due to the marine

generator engine’s exhaust. Because the engine is fixed in the boat, and the locations where the exhaust can infiltrate are 

known, this seemed to be a straightforward solution; however, all the primary stakeholders, including the National Institute 

for Occupational Safety and Health (NIOSH), the U.S. Coast Guard (USCG), the U.S. Environmental Protection Agency 

(EPA), engine manufacturers, and the boating industry, agreed that “one of the best approaches to CO control would be to 

reduce it at the source.” 9 These stakeholders rejected use of a CO-sensing shutoff system to address the hazard, citing that 

“Houseboats and other cabin boats are generally fitted with CO detectors . . . often when the CO detectors do alarm, users 

tend to believe that they are malfunctioning because they do not smell gas. In the case where the alarm goes off frequently, 

the user may just disconnect the CO detector, even if it has been operating correctly.” 9 As a result, the engine manufacturers 

voluntarily developed low CO emission engines rather than using a shutoff system. 

g Some manufacturers recommend, or sell with the generator, extension cords 15 or 20 feet long. Almost all manufacturers 

also recommend that generators be placed under cover when it is wet outside because, although these products are intended 
4

limit is ongoing; however, at this time, 300 g/hr is staff’s recommendation for this requirement. 

Staff is refining the details for a test method that uses a constant-volume sampling (CVS) or 

raw exhaust emission measurement system to determine the generator’s emission rate with 

each of the three loads applied to the generator at ambient O2. 

2. When operated in a test chamber, the generator shuts off before the engine emits

approximately 25 grams of CO. This is expected to protect the occupants who take no 

protective action and remain in the consequent CO exposure profiles against serious or lasting 

adverse health effects when the generator is operated in an enclosed space.

8 Although staff’s 

analysis of this is ongoing, staff is not currently considering recommending a limit lower than 

this. The corresponding test method to verify the amount of CO emitted could be performed in 

a chamber (not necessarily the one described above) and the amount of CO would be 

calculated from the air change rate, chamber volume, and analysis of the CO concentration 

data. What must be defined further for this test method, however, are the environmental 

conditions that should be set for the chamber test and the state of conditioning the shutoff 

system on the generator should be in when tested. The shutoff system must be durable and

work throughout the generator’s operational life, without calibration or service, so designs 

must consider the wide variety of environments in which consumers use their generators and 

store them during prolonged non-use periods. 

Depending on the technology employed for the shutoff system, both the usage and storage 

environments can pose significant challenges to the durability and reliability of the sensor and 

control components. For instance, between uses a consumer may store a generator for years in 

a garage, exposing the shutoff system to extreme temperatures and humidity as well as a 

multitude of contaminants (e.g. vapors and particulates) that could contaminate a sensor. 

Further, when the generator is in use, the shutoff system will be exposed to the engine’s 

exhaust, heat, and vibration as well as the extreme winter and summer ambient conditions 

which cause power outages that motivate consumers to use generators. It is important to note 

that CO sensors used in residential CO alarms are not designed for use in such extreme 

environments and CO sensors that may be more durable and used in commercial or industrial 

settings require periodic professional maintenance and calibration. The test method used to 

verify a shutoff’s performance must account for these and other conditions that the shutoff 

 

for outdoor use only, they are not weatherized for safe use in wet conditions where they can present a risk of shock or 

electrocution. Staff believes these recommendations can put the consumer at serious risk of exposure to the CO hazard 

because the recommendations may encourage generator operation in close proximity to a residence or an enclosed 

environment. During extended power outages in particular, homeowners experience a sense of urgency for basic needs, such 

as heat and food refrigeration; yet the power cord on many home appliances commonly is not long enough to reach windows 

or other openings where the consumer could connect the appliance to a 15- or 20-foot extension cord and be able at the same 

time to locate the generator far away from the opening. Although only a limited number of deaths in CPSC databases are 

attributed to exhaust entering the house from outdoor generator use, it is important to note that CPSC does not count or 

estimate CO injuries, but the CPSC databases do have records of such injuries. There are other published sources that show 

injuries from outdoor operation as well, and a number of these sources document that the injured consumers used their 

generators an average of only a few feet away from the nearest door or window. In 2013, the Centers for Disease Control and 

Prevention (CDC) began recommending that generators should never be placed less than 20 feet from an open window, door, 

or vent where exhaust can vent into an enclosed area. This recommendation is based on results of modeling studies 

performed by NIST on the effects of operating an existing, carbureted generator outdoors on indoor CO concentration 

profiles.

To put the CO emission rate of an existing generator into perspective relative to a car, the unmodified carbureted 5 

kW portable generator, when operating at ambient O2 (20.9%), has a CO emission rate that is over 200 times more than that 

of an idling 1996 Oldsmobile Cutlass. 10 That factor increases to more than 1,500 times greater when the generator is 

operating in an enclosed space. 

6

References

1. Mott, J.A., et al., National Vehicle Emissions Policies and Practices and Declining US Carbon

Monoxide-Related Mortality, Journal of the American Medical Association, 288 (8): 988-995, August 

2002.

2. 16 CFR part 1407, Portable Generators; Final Rule; Labeling Requirements, Federal

Register, 72 FR 1443, January 12, 2007, and 72 FR 2184, January 18, 2007.

3. Hnatov, Matthew, Incidents, Deaths, and In-Depth Investigations Associated with Non-Fire Carbon 

Monoxide from Engine-Driven Generators and Other Engine-Driven Tools, 1999-2012, U.S. 

Consumer Product Safety Commission, Bethesda, MD, August 2013 (Docket Identification CPSC2006-0057-0016, available online at www.regulations.gov).

4. Hnatov, Matthew, Non-Fire Carbon Monoxide Deaths Associated with the Use of Consumer 

Products, 2009 Annual Estimates, U.S. Consumer Product Safety Commission, Bethesda, MD, 

December 2012. (Docket Identification CPSC-2006-0057-0013, available online at 

www.regulations.gov).

5. CPSC staff report, Technology Demonstration of a Prototype Low Carbon Monoxide Emission 

Portable Generator, U.S. Consumer Product Safety Commission, Bethesda, MD, August 6, 2012. 

(Docket Identification CPSC-2006-0057-0002, available online at www.regulations.gov).

6. Public comments on CPSC Staff Report Technology Demonstration of Prototype Low Carbon 

Monoxide Emission Portable Generator – dated September 14, 2012, U.S. Consumer Product Safety 

Commission, Bethesda, MD, (Docket Identification CPSC-2006-0057-0004 available online at 

www.regulations.gov).

7. S. J. Emmerich, A. K. Persily, and L. Wang, Modeling and Measuring the Effects of Portable 

Gasoline Powered Generator Exhaust on Indoor Carbon Monoxide Level (NIST Technical Note 

1781), Feb 2013. (Docket Identification CPSC-2006-0057-0005, available online at 

www.regulations.gov).

8. A. K. Persily, et al., Residential Carbon Monoxide Exposure due to Indoor Generator Operation: 

Effects of Source Location and Emission Rate (NIST Technical Note 1782), June 2013. (Docket 

Identification CPSC-2006-0057-0012, available online at www.regulations.gov).

9. Samulski, Mike, Summary of Government/Industry Carbon Monoxide Workshop and Two FollowUp Meetings, March 31, 2004, (Docket Identification EPA-HQ-OAR-0008-0008, available online at 

www.regulations.gov).

10. Frey, H., et al., On-Road Measurement of Vehicle Tailpipe Emissions Using a Portable Instrument, 

Journal of the Air & Waste Management Association, Vol.53, August 2003.