What Causes Painful Red Rashes On The Chest, Back And Legs?

Hi! My husband has large dark red painful rashes developing on his skin,( chest, back, groin,and sometimes lower legs) whenever he has been outside for any length of time. He has had this develop each year in the spring and summer for the last 3 years. He has had this come on quite severely over the last week after supervising large energy development sites outdoors in wyoming. He gets some improvement from antibiotic creams and such, but if this remains like the previous 2 summers, he will deal with it till he is completely indoors again next winter. Any clues as to what is causing this, or if he should seek medical treatment? Thank you. YYYY@YYYY