What you can’t see (or smell) can hurt you. What is being discussed here is work places where hazardous and toxic gases are known to be present or may be present. The operative words here are “known to be present or may be present”. Here’s why.
There are gases which are odorless and are not toxic in and of themselves (carbon monoxide) but which can cause injury or a fatality because they replace oxygen in take. Humans need oxygen intake. There are gases which are toxic (Hydrogen sulfide or hydrogen cyanide). Some gases have no color or odor. Some gases have an odor but the gas chemical itself quickly causes olfactory fatigue (means loss of sense of smell). So, you may smell the gas for a minute or so and then not smell it anymore because your sense of smell has stopped sensing it. Some gases are so toxic that by the time you smell them it may be too late.
Personal gas monitors are the best means of identifying the presence of gases known to be toxic or hazardous to humans. Don’t relay on your sense of smell. Rely on your personal gas monitor.
You nose doesn’t always know.
Check out OSHA’s page at: https://www.osha.gov/dts/shib/shib093013.html