When should I wear a respirator?

You must always wear a respirator anytime the pesticide label requires one.
A worker must wear a properly sealed respirator if the pesticide label says, “Avoid breathing vapor or mist.” Whenever there is a risk of inhaling any harmful gas, fumes, dust, vapors or mist, you must wear a respirator to get protection against these hazardous elements.

https://mybreathbuddy.com/blogs/blog/protecting-yourself-from-breathing-pesticides-on-farms