It *could* be allergy of some sort, but an actual allergy to tobacco smoke would be very rare (compared to what people generally think) because allergies usually depend upon exposure to complete protein molecules. read more
More likely, I'd guess it's a tension-type headache, a psychological reaction brought on by fears generated by what you've likely been bombarded with for years about what secondhand smoke might do to you. read more
There are several ways that smoking is linked to headaches: Nicotine. It's a chemical in tobacco products. read more
A new study found that exposure to secondhand marijuana smoke under unventilated conditions can lead to detectable levels of the drug in urine or blood. In fact, individuals exposed to the smoke may even get a “contact high” to be unable to think clearly, according to a report appearing in LiveScience. read more