Carbon Monoxide Poisoning: Operating Fossil Fuel Engines Inside Buildings (AEN-206)

The use of internal combustion engines operating on gasoline, LPG, diesel fuel, or natural gas inside buildings presents a serious risk of carbon monoxide poisoning. During complete combustion, the typical combustion products from engines are carbon dioxide, nitrous oxides, particulates, water vapor, and numerous other contaminants. Several of these combustion products are linked to health problems.

During incomplete combustion, carbon monoxide, a deadly toxin, is produced. Carbon monoxide (CO) is colorless, odorless, tasteless, and non-irritating. The effects of carbon monoxide at low concentrations mimic common influenza and often are not recognized. High concentrations of CO interfere with thought processes complicating the diagnosis. Carbon monoxide is a cumulative poison which can rise to harmful levels in the body in minutes.

Does the concentration of carbon monoxide produced by engines vary? Yes, CO emitted from the tailpipe of engines burning gasoline, diesel, or LPG (propane) varies from over 100,000 parts per million (ppm) to less than 15 ppm. In 1968 the EPA regulated CO emissions from on-road motor vehicles. Engines used indoors were not originally regulated. Recent regulations affect small engines such as those used on lawn mowers, chain saws, weed eaters, electric generators, water pumps, and boats, although the regulations for small engines will continue to allow considerably higher CO concentrations than the tighter regulations for on-road motor vehicles.

Why does the concentration of CO produced vary? Carbon monoxide is produced during incomplete combustion. Anything that leads to incomplete combustion increases CO production. Two major causes are a rich fuel mixture (more fuel than is needed), or restricted air supply (dirty or plugged air filter). A gasoline engine producing 10,000 ppm CO at the ideal air-fuel ratio will produce over 60,000 ppm when the fuel is increased. Other causes of high CO production include; a cold engine, misfiring, incorrect engine timing, defective or worn parts, exhaust system leaks, and defective catalytic converters.

Why would a gasoline engine be set rich? The distribution of a liquid fuel, such as gasoline delivered through a carburetor, is not uniform. To ensure all cylinders obtain sufficient fuel to produce maximum power, the mixture must be rich. Excessively lean mixtures can cause engine problems, although fuel economy is improved with a lean mixture. Combustion analysis equipment showing the air/fuel ratio and/or carbon monoxide production assists in correctly tuning an engine. Tuning by “sound” and “performance” more likely will produce an excessively rich setting, with higher CO concentrations. Improvements in fuel delivery systems, such as fuel injection coupled with oxygen sensors in the exhaust stream, greatly improve the control of the air/fuel mixture, improve fuel economy, and reduce carbon monoxide production.

What do catalytic converters do? The three-way catalytic converter reduces the amounts of nitrogen dioxide, hydrocarbons (unburned fuel), and carbon monoxide. CO concentrations as low as 15 ppm have been measured from a gasoline engine with fuel injection and a catalytic convertor.

Is carbon monoxide a problem with diesel engines? Usually not, although any engine, including diesel, produces CO when combustion is incomplete. Diesel (compression ignition) engines run with an excess of air and often produce less than 1200 ppm CO. When diesel fuel is burned incompletely or when overloaded and over-fueled (rich mixture), diesel engines will produce high concentrations of CO. Diesels usually pollute the air with particulates and nitrogen oxides, not CO.

Is carbon monoxide a problem with LPG engines? Yes, and the same precautions against running a gasoline engine in an enclosed space should be observed with an LPG engine. Industry sources report a properly tuned LPG engine will produce from 200 to 20,000 ppm, depending on load. A difference in CO production from an engine operating on LPG and one operating on gasoline usually results from more complete combustion of the LPG because it is already a vapor. Unfortunately, most LPG engines have simple fuel delivery systems which can easily be adjusted too rich, allowing extra fuel into the engine and the subsequent high production of carbon monoxide. On one new engine, adjustment of the idle mixture reduced CO concentrations from 44,500 ppm to 600 ppm.

If LPG engines can produce high levels of CO, why are they used inside buildings? LPG burns cleaner than gasoline, and is a common fuel for forklifts and other engines used inside. The exhaust fumes are noticeably free from aldehydes, the odorous and eye-irritating compounds found in gasoline exhaust. Typically LPG engines produce less carbon monoxide than a straight gasoline engine, however new modern gasoline engines with catalytic converters and fuel injection, will produce less CO than an LPG engine. Remember that LPG engines do produce CO, and LPG engines running rich or misfiring produce extremely high concentrations of CO. NEVER USE LPG ENGINES IN AN UNVENTILATED AREA!

What about other engines used inside, like those on gasoline powered electrical generators, concrete finishers, water pumps, and high pressure power washers? Small gasoline engines used on many tools typically use simple carburetor systems with limited control over the air-fuel ratio. The engines run rich with high concentrations of carbon monoxide, typically 30,000 ppm or more. Manufacturers stress that the engines are to be used only in well-ventilated outdoor areas, and are NEVER to be used indoors even with ventilation. A 1996 National Institute for Occupational Safety and Health Alert calculated carbon monoxide concentrations in a 10,000 cubic foot room (21 x 21 x 21 feet) when a 5-horsepower gasoline engine was operated. With one air change per hour CO concentrations reached over 1,200 ppm (the Immediately Dangerous to Life and Health level) in less than 8 minutes. Even with ventilation providing 5 air changes per hour, 1200 ppm was reached in less than 12 minutes. It is not safe to operate gasoline engines indoors!

Are LPG powered floor buffers safe to use indoors? The combustion pollutants produced are a potential health risk and are known to have caused carbon monoxide poisoning. Special engines with oxygen sensors and catalytic converters which closely control the air-fuel ratio and reduce contaminant concentrations (including carbon monoxide) in the exhaust stream are available. Only buffers with low emission engines should be used indoors. Manufacturers’ recommendations must be followed; provide adequate ventilation, proper maintenance, training for workers, and using carbon monoxide detectors. Remember that high risk individuals, such as the elderly, the young, and the sick are at special risk of carbon monoxide poisoning.


Prepared by
T.H. Greiner, Ph.D., P.E.
Extension Agricultural Engineer

The Iowa Cooperative Extension Service’s programs and policies are consistent with pertinent federal and state laws and regulations on nondiscrimination regarding race, color, national origin, religion, sex, age and disability.

File: sep98\AEN-206