Hi. A Friend Of Mine Has Been Getting Hives All

Hi. A friend of mine has been getting hives all over his body every day for over a year now and the dr's can't pinpoint it. Had over 75 different allergy tests done and all come back negative. Has been taking a daily journal of everything he eats, drinks, or comes in contact with for 3 months now. This all started about a month or two after moving into the house he is renting. Could it possibly be caused by some type of mold issue or worse yet prior meth production in the home?