I am a truck driver local. as of last year we were issued company shirts. they have reflective stripes swonek?

i am a truck driver local. as of last year we were issued company shirts. they have reflective stripes and do not breath. my manager says even with a doctors excuse i have to wear it. when the temp... is 90 or more i get sick ,vomit, and weak. i have had heat stroke twice and am very susceptible to heat. there are two more drivers that have the same issue. and the temp drivers don't have to wear them. i have been fighting this since it has gotten hot and i am being called into the office tomorrow. pleas help.