Good ol' olfactory
A candle that smells good to most people. A blossoming tree that everyone else wants to sniff. Perfume that your friend wears one teensy spray of. Very diluted bleach you use to clean your house. A whiff of cigarette smoke as you hug a friend goodbye.
All of these things can trigger a migraine, and one of them is making me feel sick on an otherwise healthy day.
Can you tell when a migraine attack is coming?