Airbags deploy when a moderate to severe crash impact triggers the electrical control mechanism that inflates the bags. More advanced airbag computer systems use crash sensors, seat belt sensors, weight sensors in seats and car speed to determine exactly when to deploy.
When triggered, an igniter fires and sets off a chemical reaction that quickly produces a gas that inflates the airbags. Airbags inflate in less than half a second in frontal crashes.
Airbag computers are programmed with must-deploy speed thresholds to ensure that they only deploy when needed to prevent injury. These thresholds take into account readings from sensors. For example, if front passengers are wearing seat belts, the deploy threshold for a frontal crash is approximately 16 mph.