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A smoke warning has been issued for the Perth metropolitan area and coastal areas south of Perth because of 20 South West fires.

The Department of Fire and Emergency Services said smoke from more than 20 bushfires had built up over the past two days.

Motorists driving through smoke were urged to turn on their headlights and drive cautiously.

People with conditions that could be made worse by smoke inhalation should take suitable precautions.

The smoke was expected to clear with the sea breeze this afternoon, but DFES said it could thicken again overnight.

"The smoke is being trapped under an inversion that acts as a 'blanket', keeping it relatively closer to the ground," the statement said.

"This inversion prevents the smoke dispersing until temperatures rise and 'punch a hole' in the inversion layer and the surrounding atmosphere becomes more unstable."