Are diesel generators a health hazard?

It would depend on what type of hazard is being considered.

If the generator is very large, and poorly fitted, it might fall on you.

If the wiring is not adequately shielded/earthed, it might electrocute you.

If the location is poorly ventilated, it might poison/suffocate you.

If the moving parts are not guarded, there is a danger of getting caught in these moving parts and injured.

SO yes, diesel generators are "a health hazard" under some situations.

No. First diesel fumes are non toxic. Most generators are placed where the average person cannot get access to them. Rebuttal.

Diesel Fumes are not toxic? Never heard of "carbon Monoxide" then I guess?

All combustion engine exhaust is an asphyxiate as it is lacking in oxygen, as the oxygen has been used in the combustion of the fuel. If the engine is not properly maintained it's also very probably it produces toxic gasses as well. Such as carbon Monoxide.

To say "diesel fumes are non toxic" is ignorant at best, and dangerous at worst.

Also, it's an assumption by the poster of the above that the engine has been fitted in a place where the "average person can not access it."