While a forklift lift contributes to CO emissions other items do too - like gas heaters. It is the concentration of CO concentration in the ambient air in the working environment you need to be concerned. Last I recall it was a max of 50 ppm per OSHA regulations - could be less today as I've been out of this business since '.07. The air exchange system ventilation system can be the difference in a pass or fail "CO" sniffer test and can vary significantly based on how full or empty the warehouse is and how long the forklift operates and other things

Every winter when the temperature drops in Australia people rush to switch on their central ducted gas heaters or other gas appliances. But it might be an invitation to a silent killer (CO) as it increases the potential risk of CO poisoning if the gas heater is faulty. https://ductscleaning.com.au/service/carbon-monoxide-testing-service/

The air exchange system in the building has a lot to do with the total CO concentration in any building too plus other sources of CO emission.
Hard to believe that any brand of lift has twice as much as another brand as they all must comply to the same Tier standards - can't recall seeing this info on a spec. But right now right now many popular Toyota LP lifts shipped have zero emissions - they are all in dry dock last I heard.

IC truck emissions should always be a concern for companies that run IC trucks indoors on a consistent basis. It is up to the customer to make sure they monitor the CO levels and keep their machines maintained to insure the CO levels are kept to an acceptable level. Proper building ventilation is also a factor.
As for as which brand lift has more CO emissions versus another? As i said previously, it all depends on the maintenance and the consistency of monitoring the trucks emissions and making sure they are tuned up and CO levels kept down to a minimum. Sure, if there is a real concern about CO emissions then install CO detectors to monitor this throughout the areas effected.

read and become enlightened. Found this info in less than 2 minutes via a Goggle search. The last I knew Canada adopts many of the OSHA regulations but has a hard time getting Lord Stanley's Cup North of the US border - 1993 last time as I recall - but we still appreciate having Gordie Howe in Hockey-town USA - grew up in the Motown area where we drive SOUTH to get to Canada. Ciao!
https://www.epa.gov/indoor-air-quality-iaq/carbon-monoxides-impact-indoor-air-quality
https://www.homedepot.com/p/UEi-Test-Instruments-Carbon-Monoxide-Detector-with-Alarm-CO71A/306091460?source=shoppingads&locale=en-US&&mtc=Shopping-B-F_D27L-G-D27L-027_005_FIRE_SAFETY-NA-NA-Feed-SMART-NA-NA-SAFETY_SECURITY_FireSafety_New_Engen&cm_mmc=Shopping-B-F_D27L-G-D27L-027_005_FIRE_SAFETY-NA-NA-Feed-SMART-NA-NA-SAFETY_SECURITY_FireSafety_New_Engen-71700000082263410-58700006984682162-92700062875449987&gclid=EAIaIQobChMIjLPS8Nfs8wIVxaeGCh20pQD5EAQYBCABEgKAFfD_BwE&gclsrc=aw.ds

No Idea about the CO level

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.