Earlier this year, I wrote an article titled “What type of respirator do I need?” in which I explained how respirator users and administrators can use data to determine the proper type of respiratory protection that would be needed. But this article was written under the assumption that the user/administrator had the most essential piece of data… the level of the contaminant. This article helps explain what OSHA says in their May 2012 Letter of Interpretation regarding air sampling and respirator use.