When eating a spicy curry, you often start sniffling after a few bites. What is going on?
Most spicy foods contain capsaicin or allyl isothiocyanate. Capsaicin is the active component of chili peppers, whereas allyl isothiocyanate is a colorless oil that can be found in things like mustard, radishes, and wasabi.
These chemicals are responsible for a burning sensation on the tongue, but also irritate the mucous membranes in your nose, causing them to become inflamed and produce extra amounts of mucous. Thus, your body tries to keep out the substance or particle causing the irritation.
In the same way, capsaicin and allyl isothiocyanate can irritate the membranes in your eyes. This triggers your tear ducts to produce extra tears, to wash away the irritant. As a consequence, tears may drain into your sinuses, causing your nose to run even more.