That's because benzene and other fumes from car exhaust could be entering your house.
Benzene is a volatile organic compound, or VOC, that’s found naturally in crude oil and thus in gasoline and vehicle exhaust.
There are already low levels of benzene in the air all around us due to air pollution from motor vehicle exhaust.
Researcher's from Health Canada’s indoor air section has conducted studies measuring levels of the gas in homes across Canada.
Benzene levels in houses with attached garages were found to be around three times higher than of other houses without garages.
And even after a car is turned off, the engine will continue to emit benzene into the air as it sits in the garage.
Paints and solvents that many homeowners store in their garage may also emit benzene as they slowly evaporate.
Health Canada, the World Health Organization, and the European Commission recommend that people reduce their exposure to benzene as much as possible.
Studies have shown that benzene can definitely cause problems if people are exposed to high levels over long periods of time.
Workers in industrial settings exposed to high levels of benzene have been shown to have a much higher risk of leukemia.
Benzene is dangerous because of the damage it can do to the blood.
It causes bone marrow not to produce enough red blood cells, while also damaging the immune system by not creating enough white blood cells.
Homeowner are advised to never idle a vehicle inside a garage, but to let it warm up outside.
Other recommendations for minimizing the transfer of garage air to the home include:
Make sure the weather stripping around the door to the garage is continuous and in good shape;
Have spray foam insulation installed to seal the wall between the house and garage. Then drywall can be installed over top to further reduce air leakage;
A similar approach can be taken to seal the ceiling space between the garage and any rooms above. This will also help reduce energy costs and keep the floors warmer; and
Another approach involves installing an exhaust fan to vent garage air to the outside. The fan would also help depressurize the garage relative to the house, thereby preventing air movement from the garage to the house, even if leaks exist.