Correction: A previous version of this story misstated the number of states under red flag warnings.
Five states were under red flag warnings Tuesday due to hot, dry and unstable conditions, the National Weather Service said.
Parts of Pennsylvania, New Jersey, Michigan, Washington and Alaska will remain under the warning until 8-10 p.m. local time, the weather service said.
In Pennsylvania, lightning strikes may cause fires to start in dry areas. In Washington, dry and unstable air mass with warm temperatures could contribute to a fire.
“Be careful with any activities that could potentially lead to a wildfire,” the weather service warned Michigan residents. “Camping, outdoor grills, smoking materials, chain saws and all-terrain vehicles all have the potential to throw a spark and ignite a dangerous and destructive fire.”
What is a red flag warning?
The weather service issues a “red flag warning” when weather conditions in an area are ripe for the spread of wildfires due to a combination of strong winds, low relative humidity and dry fuels.
“Any fires that develop will likely spread quickly,” the weather service said. “Outdoor burning is not recommended.”
Wildfires in Canada affecting air quality in US
The weather service issued air quality alerts to several U.S. states warning “sensitive groups” such as older adults, children and those with preexisting respiratory conditions like asthma to limit prolonged or strenuous outdoor activity.
Air pollution increases the risk of respiratory infections, heart disease and lung cancer, according to the World Health Organization.
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This article originally appeared on USA TODAY: Wildfire, smoke map of US, Canada: Red flag warnings in 5 states