Rockingham, Mandurah and surrounding areas were blanketed by smoke overnight.
The smoke was due to prescribed burns being carried out by the Parks and Wildlife Service.
A smoke alert was issued for Busselton, Donnybrook, Bunbury, Mandurah and the Perth metropolitan area on Tuesday night.
The smoke is expected to clear this morning.
WHAT TO DO:
• If the smoke is particularly thick, it is recommended people 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.