It is illegal for your employer to let you get away with not wearing your respirator when required.

The answer is yes, it is illegal. It is because the employer is the one responsible of having his or her employees' safety especially when working. It is his or her job, in making sure that his or her employees have been doing their job right and having to have a respirator when needed is a way of the employer to tell his or her employee to have his or her respirator to be worn.