I'm writing a business plan for robots that work in dirty or dangerous
environments, and I need a statistic for the number of people who work
with respirators or chemical suits, or the number of such protective
gear sold annually. I'd like to say something like "XXX workers in the
US wear respirators in their daily job function." From a reputable
source please, like OSHA or an insurance company, or a large
manufacturer of protective gear.

Hello tlbtlbtlb and thank you for your question.

You can now say "5 million workers in the US wear respirators in their
daily job function." (OSHA)

"OSHA estimates that more than 1 million people regularly use
respirators, because they work in environments that contain toxic air
contaminants or are deficient in oxygen. Almost 2 million more workers
occasionally use respirators, while another one-half million use
respirators routinely and for emergencies."
http://www.environmental-center.com/articles/article1051/article1051.htm
(Total 4.5 million)

"OSHA's Respiratory Protection Standard covers an estimated 5 million
respirator wearers  working in an estimated 1.3 million workplaces in
the covered sectors."
http://www.osha.gov/SLTC/respiratoryprotection/

There is also a full market survey regarding protective equipment. 
However it is rather expensive at US $3650 (Hard Copy) US $4200 (PDF
by E-mail)
http://www.the-infoshop.com/study/bc11898_protective_clothing_toc.html

Thank you for your question and if you need any clarification of my
answer, do not hesitate to ask.

Very best regards.

THX1138

Search strategy included
"use respirators" million site:osha.gov
://www.google.com/search?hl=en&lr=&ie=UTF-8&oe=UTF-8&as_qdr=all&q=%22use+respirators%22+million+site%3Aosha.gov&btnG=Google+Search

Just what I wanted to know. Thanks.

Hello again tlbtlbtlb.

Thank you very much for the excellent rating and nice tip!

Regards

THX1138