The family had purchased a bottle of CO2 for an aquarium and were travelling home on the A2 motorway near Lucerne just before midday on Saturday, according to Lucerne police.
Gas began to leak from the defective bottle and filled the car, causing all four to fall unconscious.
Shortly before the Sonnenberg tunnel the car slowed and came to a halt, then rolled backwards into the fast lane.
For several minutes traffic flowed past the vehicle until someone stopped and saw all four occupants of the car were unconscious.
After the doors were opened, they all gradually came round and were taken to hospital.
The family were very lucky not to have suffocated in the car, said police.
According to 20 Minutes the family were from the canton of Obwalden and included two children aged six and ten.