Here is what OSHA states
"OSHA PEL] The current Occupational Safety and Health Administration (OSHA) permissible exposure limit (PEL) for carbon monoxide is 50 parts per million (ppm) parts of air (55 milligrams per cubic meter (mg/m(3))) as an 8-hour time-weighted average (TWA) concentration [29 CFR Table Z-1].Oct 22, 2020"

In a nut shell having a sufficient building air exchange system can help business to be in compliance no matter what the forklift lift CO out put.

Of course, all forklifts must comply to the same emissions EPA standards Tier II, III, IV since the early 2000s depending on the year of manufacture.