In April 2013 BP affected the left side of my face. That August, as the left was starting to recover, it hit the right side. Things didn't improve. Blood tests and a cerebrospinal fluid check couldn't find any possible cause so it just looked like bad luck. I recently had my gas boiler replaced. The fitter was horrified and showed me that the old boiler had been belching out carbon monoxide. I hadn't bothered to get it serviced! My house is quite open plan and this may have lessened the exposure. Since the boiler has been replaced the headaches and general feeling of being unwell have gone and my face is starting to improve. If you can't find any other cause, and you have a gas boiler, I suggest you get it checked.