The average carbon monoxide concentrations (exhaust gases) for well-maintained *forklifts (lift trucks, Hi-Los) are as follows:
also see data for Other Combustion Sources
Opinion: Based on the data above, diesel, propane, and gasoline-powered forklifts should NEVER be used indoors (warehouses, factories, etc.) unless adequate exhaust purification has taken place, or unless they are powered by electricity.
Manuals for the maintenance of LPG (propane) powered forklifts state that with proper adjustment and with adequate ventilation, mechanics should be able to keep the CO level below the OSHA limit (35 ppm measured over an 8-hour period of the work). The manuals strongly recommend the use of a CO analyzer in the tuning procedure.
Nevertheless, it is this webmaster's opinion that in most cases it is not possible to keep air CO concentration below the OSHA limit where several forklifts are operated simultaneously even in the relatively large volume of a warehouse. The large number of cases of CO poisoning produced in this manner, as reported directly to me and as noted in the literature (see MMWR, 1998), suggest the correctness of my statement.
Also See Fawcett et al., 1992 Study