What factors contribute to the spiciness of onions?

The spiciness of onions is mainly due to the presence of sulfur compounds, particularly allyl isothiocyanate and syn-propanethial-S-oxide. These compounds are released when onions are cut or crushed, stimulating the nerve endings in the mouth and causing a spicy sensation. Other factors such as the onion variety, growing conditions, and storage can also influence the spiciness.