The government is urging people to avoid short car journeys where possible after it issued its first smog warning of the summer this weekend.

Defra said the combination of warm temperatures and low winds was expected to result in high ozone levels over England and Scotland over the weekend with conditions likely to persist for as long as the current heat wave continues and wind speeds remain low.

Ground-level ozone is formed when sunlight acts on nitrogen dioxide and other atmospheric substances close to the ground. The pollutants that cause ground-level ozone come from a range of sources, including petrol and other fuels. High levels of smog can contribute to respiratory problems and often prove particularly troublesome to the elderly and those with asthma.

Defra advised those sensitive to ozone to avoid exercise outdoors, particularly in the afternoon, and urged people to avoid short car journeys where possible. It further warned that drivers should avoid overfilling their petrol tanks and spilling petrol as this evaporates and releases toxic hydrocarbons that form ozone.

A spokeswoman for Defra also advised commuters stuck in traffic to turn off their engines where possible to help limit the build-up of smog. "If you need to drive and are stuck in a traffic jam, it makes sense to turn off your engine," she said, adding that traffic congestion was a particularly bad source of smog as emissions concentrate in one area and are not as easily dissipated.