Environment regulators issued a high-pollution advisory and ban on wood-burning fires for Monday in metropolitan Phoenix.
It marks the fourth consecutive day that people in Maricopa County have been told not to burn wood.
The aim is to reduce pollutants at a time when the area's pollution levels may exceed the federal health standard for air quality.
Open burning and use of wood burning fireplaces are generally prohibited.
People with respiratory problems are advised to avoid heavy or prolonged outdoor exertion.
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