Perth woke to a thick blanket of smoke as the result of a prescribed burn off at Beelu National Park in Helena Valley on Thursday morning.

The Department for Parks and Wildlife issued a smoke alert for the Perth metropolitan area, the Perth Hills and the southern suburbs.

The department advises to shut doors and windows and turn off air-conditioners.

Smoke may reduce visibility on some roads and motorists should take care, turn on headlights and travel at appropriate speeds when travelling in these areas.

People with asthma and pre-existing respiratory or cardiovascular illnesses should follow their pre-prepared treatment plan.

People with conditions exacerbated by smoke should take precautions in line with their medical advice for these circumstances.

The smoke is expected to clear later in the morning.

Originally published on WA Today as Perth wakes to smoky haze covering the metro area.