Even switched off car engines pollute the environment: Study

Even if you leave your car at home and catch a bus to work you're doing no good to the environment. A new study says vehicles left in the sun for days at a time can emit damaging hydrocarbons - one of the main ingredients in smog.

According to the Second National In-Service Emissions Study, hydrocarbons are in the vapour that escapes from petrol tanks on a warm day. Most newer cars have canisters that trap them before they are released but if cars are left sitting for longer than 24 hours the canisters can fill up and stop working until the vehicle is driven.

"The results indicate that when vehicles are parked in warm conditions for an extended period (more than a day), the evaporative emission control systems may not be able to effectively control the build-up of evaporative hydrocarbons, as even the latest systems are only designed to provide effective control for a continuous 24-hour period," the report said.

Smog is linked to respiratory problems such as asthma. It also contributes to global warming, reports The Sydney Morning Herald.

"It means for the private individual who is doing the right thing and walking and catching public transport and not using their car, they're actually causing emissions which they don't want to do," said Bruce Jeffreys, co-founder of car share company GoGet.

To reach the conclusion, researchers tested cars' evaporative emissions one hour after they were driven and then during a stationary test, where vapours were measured from each vehicle over 24 hours.