Three other factors need to be considered wherever forklift emissions are of concern:

1. Is your LPG fuel composition regulated? If it is propane what is the limit to the amount of other hydrocarbons? If it's a propane-butane mix what ranges of those gases and other gases are allowed. In Australia LPG for forklifts is an unregulated propane - butane mix. As a result any engine without an O2 sensor and a mechanism to adjust the air fuel ratio can have very high CO outputs as gas bottles are changed. In testing of mostly new or near new forklifts the CO readings were from 550 ppm to ~90,000 ppm (this was outside the instruments accurate range) with a median of 4000 ppm. At 90000 ppm it would take 3 minutes for the CO level in a 50 m long by 20 m wide by 5 m high enclosed space to reach the 8 hour CO exposure limit of 50 ppm - and in 11 minutes the maximum allowed indoor level of 200 ppm. Even at 4000 ppm the times are 1 hour and 4 hours.

2. How well tuned are your forklift engines? In the same testing above, a gasoline engine forklift (and gasoline specifications are very precise here) gave a reading of 22,000 ppm! Obviously the engine was either very much out of tune OR the emission management system was not working.

3. Do you have forklifts with engines with catalyctic converters in situations where forklift use is intermittent? In this testing it was shown that such engines took 13-14 minutes at idle to get too 500 - 800 ppm with the catalyctic converter working. But along the way CO levels peaked at 6,500 - 13,500 ppm. So if the forklift is used intermittently so the catalyctic converter rarely reaches full operating temperature you still have a problem.

Finally, in this study, the engines with consistently the lowest CO emissions were DIESEL engines with levels below 100 ppm. It would take 50 hours of operation to get to 50 ppm in the warehouse above.

In addition a diesel engine produces 30% less heat than an LPG engine doing the same task because the diesel engine is much more efficient.

So in a situation where you had to use an IC engine in a freezer for a short time, diesel would be the best choice