Consider the equation relating impulse (change in momentum) to force:
F = Δp/Δt
The average force exerted on your body when you are slowed by an airbag is your impulse divided by the time it takes to slow you down. The airbag can't change your initial momentum in a collision, but where they can vary in the time interval it takes to stop you.
Airbags in general should take some time to slow you down, resulting in a force exerted on your body that will probably be uncomfortable but nothing life-threatening. If the airbag stops you in a short time interval, the force exerted on your body can be quite high, which can lead to injury.
