Industrial production and consumption of ammonia is the basis of the fertilizer industry. There's not many hazardous chemicals we have more experience in handling safely in bulk than this stuff.
In ammonia incidents, the industry with the highest frequency of injured persons was manufacturing of food, textile, and apparel (451 [39%] of 1,153 injured persons). The subsector food manufacturing had the highest number of injured persons and agriculture, forestry, fishing, and hunting had the second highest frequency of persons injured from ammonia releases.
-- "Top Five Chemicals Resulting in Injuries from Acute Chemical Incidents — Hazardous Substances Emergency Events Surveillance, Nine States, 1999–2008", Ayana R. Anderson, MPH, Division of Toxicology and Human Health Sciences, Agency for Toxic Substances and Disease Registry, CDC
https://www.cdc.gov/mmwr/preview/mmwrhtml/ss6402a6.htm
Agricultural use of ammonia is strongly characterised by use in unconfined and well-ventilated spaces. Relatively few people are involved with its use, and those who are tend to be reasonably familiar with risks and precautions. It remains one of the top five most dangerous chemicals.
Notably absent from that list are hydrocarbon fuels, despite their considerably wider use by an almost completely untrained cohort.
Widespread use as a fuel would likely increase risk and exposure to ammonia, as well as its use within confined spaces in which the likelihood and impacts of accidental exposure would be greatly increased.
Sure, but carbon monoxide is a product of burning fossil fuels, so the fossil fuel cycle has it's poison dangers.
CO has a lower possibly lethal concentration of about 200 ppm (with long exposure), though higher levels will cause more rapid and severe issues, with levels from 800 to 12,800 resulting in death in from 2--3 hours to a few minutes. Auto exhaust concentrations without a catalytic converter are 35,000 ppm.
https://www.abe.iastate.edu/extension-and-outreach/carbon-mo...
In the case of ammonia, it is the fuel itself which is toxic.
LC50 (lethal concentration, 50% mortality) varies by duration of exposure, from 40,300 ppm (10 min) to 2,000 ppm (4 hr). LC50 for 30 minutes exposure ranges from 200--2,000 ppm.
https://www.cdc.gov/niosh/idlh/7664417.html
Note that the kill mechanisms differ markedly. CO binds to hemoglobin, preventing oxygen from doing so, and effectively asphyxiating the victim. Treatment with oxygen should promote recovery in most cases. Ammonia is chemically caustic, exothermic when hydrating, and chemically induces necrosis and tissue death, most notably in the lungs if inhaled. Treatment and recovery are far more critical than with CO.
https://www.ncbi.nlm.nih.gov/books/NBK546677/?report=classic
On top of that, CO is a fractional exhaust product whilst ammonia is the hyperabundant fuel. Any fuel spill or partial combustion presents and immediate and severe inhalation threat.
It's best to avoid both. Ammonia strikes me as the markedly greater risk.
With exposure to organic combustion virtually universal, CO poisoning deaths in the US range about 438/yr (1999--2012), or 1.48/million. From the accompanying map of case intensity, I'd surmise indoor heating is a principle risk case, with cases concentrated in northern and Rocky Mountain states. On balance, that's a fairly low per-person risk, the overall incidence rate is a measure of net exposure.
https://pubmed.ncbi.nlm.nih.gov/26032660/
In my earlier comment, CO has nearly exactly twice the incidents as ammonia, though I strongly suspect that this is based on a vastly larger potentially-expose population. Even with very limited exposure to potential poisoning, ammonia represents an absolute risk half that of CO, which has universal exposure.