LOS ANGELES — Massive wildfires burning in the Los Angeles area have filled the air with a thick cloud of smoke and ash, prompting air quality advisories across a vast stretch of Southern California.

Three major fires broke out Tuesday amid dangerously high winds, killing at least five people and destroying more than 1,000 structures. Tens of thousands of people have been told to evacuate, many in harrowing conditions.

In Altadena where one of the major fires raged, the smoke was so thick a person used a flashlight to see down the street. A dark cloud hovered over downtown Los Angeles and smoky air and ash drifted well beyond the city to communities to the east and south.

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What Are the Risks?

Wildfire smoke increases tiny particles in the air known as particulate matter that can be harmful to people’s health. Children, the elderly and people with conditions such as heart and lung disease are more sensitive to the effects.

Dr. Puneet Gupta, the assistant medical director for the Los Angeles County Fire Department, said wildfire smoke is known to cause heart attacks and worsen asthma, and that burning homes can also release cyanide and carbon dioxide. He said sickened patients are showing up in emergency rooms when hospitals already are full because of flu season, and some hospitals could also face evacuations due to the fires.

“We have a number of hospitals that are threatened, and if they have to be evacuated, it could become a crisis,” said Gupta, also a spokesperson for the American College of Emergency Physicians. “So that is one of the things that we have to consider.”

U.S. Health Secretary Xavier Becerra raised concerns Wednesday about the smoke’s impact on people’s health in the aftermath of fires that have charred massive amounts of vegetation and buildings.

“That air that’s being spewed is no longer just the kind of smoke that we used to see from wildfires, where it was natural vegetation that was burning,” said Becerra, a former California Attorney General. “Now you got a whole bunch of toxic materials that are getting burned and put into the air.”

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What Areas Are Affected?

About 17 million people living across Southern California are covered by smoke and dust advisories issued for the three wildfires, according to the South Coast Air Quality Management District.

The smoke advisory was expected to last until late Thursday. A dust advisory was also in effect until late Wednesday as gusty winds could kick up ash and dust from prior fires and further worsen air conditions, the district said.

The worst conditions were in the vicinity of the fires with some areas covered in thick, gray smoke. In East Los Angeles, the air quality index hit an unhealthy 173. Good air quality is considered to be 50 or less.

But dozens of miles away, air quality also was deemed unhealthy for sensitive groups including the elderly and young children. Officials in the city of Long Beach about 20 miles (32 kilometers) south of Los Angeles warned residents to take precautions due to the smoky air, and in coastal Rancho Palos Verdes the air quality index measured 108, which is considered unhealthy for those sensitive to pollution.

Winds from the northwest were expected late Wednesday and Thursday to push air from the regions where fires were still burning toward the south across Los Angeles and Orange counties and east toward San Bernardino County.

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What Can People Do To Protect Themselves?

People living in areas affected by wildfire smoke should try to stay indoors and keep windows and doors shut to limit their exposure.

They should avoid vigorous physical activity and run air conditioning or an air purifier, and should not use house fans that draw in outside air.

For those who must be outside, a respirator mask can offer some protection, according to air quality regulators.