Started sending my pants and shirts to INSECT SHIELD probably 5 years ago. they treat with permitherion. When I started, they said 50 washings.  It's higher now. It lasted longer than 50 washing I can assure you. 
  Yes, I was concerned about exposure, but I was more concerned about tick bits. My first 55 years, I'm guesing maybe a dozen tick bites.  Then, it went to 6-8 a year, maybe more. 

Insect shield took care of issue, military uses clothes treated with permitherin.
Keep watching this thread, I'm betting Mainefarmer posts...