Controlling Forklifts' Exhaust Emissions

The adverse effects of occupational carbon monoxide (CO) exposure from the exhaust gases of propane and liquefied petroleum gas (LPG)-fueled equipment and vehicles are well known.1,2,3,4,5,6,7 Three incidents in Iowa in a single year (1998) were typical cases. In one event, warehouse workers reported symptoms of headaches, dizziness, and nausea for several days. The three LPG-fueled forklifts operated in the warehouse were determined to have exhaust CO levels from 40,000 to 70,000 ppm, resulting in ambient CO levels above 250 ppm in the occupied warehouse areas. Another incident that year resulted in 10 employees of a plastic manufacturing plant requiring hospital treatment for acute CO toxicity, after which it was determined that the exhaust gas of the two LPG-fueled forklifts used in the occupied area contained more than 40,000 ppm CO. In a third incident, an LPG-fueled forklift with CO levels of 75,000 ppm in the exhaust produced ambient air concentrations of 200 to 450 ppm in a pallet manufacturing plant.8

A study of CO-related violations from 1994-1999 completed by the Washington State Industrial Safety and Health Administration found that LPG-fueled forklifts accounted for 25 percent of the reviewed inspections (36 of 142 inspections), with most of the CO exposure complaints originating in warehouse operations.9 According to the U.S. Environmental Protection Agency (EPA), LPG-burning forklift trucks account for more than 60 percent of all material handling vehicles and more than 80 percent of all internal combustion forklifts used worldwide.10 There are more than 500,000 LPG-powered forklifts operated in the United States that consume approximately 15 percent of all LPG produced in this country.11

In principle, however, these LPG-fueled engines needn't be a major source of CO exposure because complete combustion of fuel in the presence of an adequate oxygen supply will result in the formation of only carbon dioxide and water. However, an insufficient amount of oxygen will cause incomplete combustion of fuel and allow the formation of CO and carbon particulate (soot).12 Adjusting the air and fuel mixture can control engine CO exhaust emissions in the exhaust, as well as ensuring proper engine power and fuel consumption. At the ideal air-fuel ratio, the CO concentration in the exhaust gas of an LPG-fueled engine will be as little as 0.4 percent (4,000 ppm), an amount that may cause little concern if a reasonable amount of dilution ventilation is present.

However, if the amount of air in the mixture is only 90 percent of ideal (what is called a "rich" mixture), the CO concentration will go up by more than a factor of 10 (44,000 ppm).12 Federal OSHA regulations don't specify the allowable emissions in forklift engine exhaust, but California OSHA regulations limit emissions to 1.5 percent CO for the larger engines (above 140 cubic engines), with somewhat higher limits for smaller engines.13

Proper engine tune-ups are thus critical to ensure the exhaust gas isn't spewing out an excessive amount of CO. Unfortunately, though, the tune-up process doesn't often include measurement of the actual CO emissions, and tune-up technicians may not be trained to properly measure CO even if they are required to do so. The tune-up adjustments may therefore be set for greater power rather than minimal CO emission.14

Field Study

A field study to assess the process and effect of forklift engine tune-ups was conducted in an operational warehouse with CO exposure from five LPG-powered forklifts.

The majority of forklift operation in the facility, located in central California, typically occurs Monday through Friday beginning at 8 a.m. and ending at 4:30 p.m. The warehouse (121,000 square feet) is operated primarily as a storage and transfer facility for the shipment of bulk pallets of industrial products. There are 14 dock doors that provide tractor-trailer access for the loading and unloading of palletized consumer materials, and each dock door is manually closed when the dock door is not being used to access a trailer. The warehouse does not have a mechanical ventilation system, but there are 32 passive skylight vents in the warehouse roof.

The forklifts are equipped with electronic emission and fuel computer modules to provide desired air-fuel ratios by continuous, active control of the fuel power valve, fuel injection system, and emissions sensors. The study forklifts were purchased as new models three years before the study and undergo tune-up and maintenance on a monthly basis by trained technicians. However, the tune-up process at this facility did not customarily include measurement of the CO emissions, as is typical for most forklift tune-ups.

Before a tune-up, CO measurements were made in the exhaust gas of each of the five forklifts using both colorimetric (detector) tubes and a non-dispersive infrared (NDIR) gas analyzer and a commercially available tailpipe probe to sample and cool the exhaust gases directly from the tailpipe. The exhaust gas CO concentration averaged 5.0 percent for the five forklifts before the tune-up (range 3 to 7 percent), even though these were up-to-date engines that were regularly tuned. Both measurement methods gave the same results, but the NDIR analyzer was much easier and faster to use. It also required less operator care to ensure accurate results, though periodic calibration is necessary.

During the tune-up, the technician used the NDIR analyzer to aid in adjustment of the air-fuel ratio in a much more precise way than the customary process. At the end of the tune-up, the CO concentration in the exhaust averaged only 0.82 percent (range 0.7-0.9 percent), a reduction of more than 80 percent from the original concentration. The emissions control system of the forklifts was found to maintain the same CO output at all engine speeds from idle to full power. Efforts to use detector tubes to aid in the engine adjustment were unsatisfactory due to the length of time required for a measurement with a single tube. However, they were entirely suitable for an assessment of the CO emission either before or after the tune-up.

The detector tubes also were used to sample at a location 1 foot behind the tailpipe opening, to simulate the situation in which a tailpipe probe is not available. Because of the dilution of the exhaust gas with the surrounding air, the measured concentrations were much lower, with an average of 0.4 percent (range 0.25 to 0.55 percent) CO before the tune-up. Without correction, this sort of measurement would give a very misleading estimate of the exhaust emissions. However, the ratio between the in-tailpipe readings and the behind-tailpipe readings was quite consistent for the five forklifts (average 12.4, range 11.4 to 13.3). If no tailpipe probe had been available in this case, multiplying the measurements 1 foot behind the tailpipe by a correction factor of 12 would have given acceptable estimates of the true exhaust gas concentrations (within ± 10 percent of the true reading for all five forklifts).

Before and after tune-up of the forklift engines, breathing zone and area air samples for CO were also collected in the warehouse and adjacent spaces. Eight-hour time weighted average samples for two forklift operators and two helpers averaged 25.5 ppm before the tune-up and 10.3 ppm afterward; stationary samples at three locations scattered around the warehouse decreased from an average of 17.7 ppm to 8.7 ppm; and area samples collected in work areas attached to the warehouse (employee break room, manager's office, reception) decreased from 10.7 ppm to 4.3 ppm. These reductions of CO concentration ranged from 51 to 60 percent--a substantial improvement, though less than the 82 percent decrease in the exhaust gas CO emission.


Tune-ups of LPG (or propane)-powered forklifts using CO measurement are an effective way to reduce worker exposure when the forklifts are used in occupied spaces. Of course, even this precaution may not be sufficient under conditions of minimal dilution ventilation. Electric-powered forklifts or improved ventilation may be needed in those cases for control of the hazard.

An excellent discussion of the problem and the solutions is provided by the Washington State Department of Labor and Industries at


1. Roberge, B.: Evaluation and Control of Carbon Monoxide Exposure from Propane-Fueled Forklifts. Appl Occup Environ Hyg 13(3): 183-191 (1998).

2. Demer, F. R.; Rosen, J. C. and Finman, T. J. Carbon Monoxide Exposures During the Use of Propane-Powered Floor Burnishers. Appl Occup Environ Hyg 11(8): 1087-1091 (1996).

3. McCammon, J. B. and McKenzie, L. E.: Carbon Monoxide Poisoning Related to the Indoor Use of Propane-Fueled Forklifts in Colorado Workplaces. Appl Occup Environ Hyg 11(3): 192-198 (1996).

4. Fairfax, R.: Carbon Monoxide Exposure from a Propane-Fired Concrete Cutter. Appl Occup Environ Hyg 15(5): 400-401 (2000).

5. Huffman, S. M.: Exposure to Carbon Monoxide from Material Handling Equipment. J Occ Env Hyg 1(5): 54-56 (2004).

6. DiNardi, S. R.: The Occupational Environment: Its Evaluation and Control. 4th ed. Fairfax, VA. AIHA Press, 1997.

7. Plog, B. A., Niland, J. and Quinlan, P. J.: Fundamentals of Industrial Hygiene. 4th ed. Itasca, IL. National Safety Council, 1996.

8. National Institute of Occupational Safety and Health: Carbon Monoxide Poisoning Associated with Use of LPG-Powered (Propane) Forklifts in Industrial Settings – Iowa 1998. MMWR 48(49): 1121-1124 (1998).

9. National Institute of Occupational Safety and Health: Carbon Monoxide Poisoning Associated with Use of LPG-Powered (Propane) Forklifts in Industrial Settings – Iowa 1998. MMWR 48(49): 1121-1124 (1998).

10. United States Environmental Protection Agency, Assessment and Standards Division, Office of Transportation and Air Quality: Large SI Engines Technologies and Costs. Available at

11. Willis, Roy: Industrial Truck Association. 2003 Annual Conference President's Address. Available at

12. Heywood, J.: Internal Engine Combustion Fundamentals. New York, NY. McGraw-Hill, Inc., 1988

13. California Code of Regulations, Title 8, Section 5146. Internal Combustion Engine Exhaust Emission Control. Available at

14. Whitaker, C.: Prevent Carbon Monoxide Poisoning from Forkifts (2005), Washington State Department of Labor and Industries. Available at

This article originally appeared in the January 2009 issue of Occupational Health & Safety.

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