A rapidly growing fire in the foothills of Glendora showered a wide area of Los Angeles County with smoke and ash Thursday, prompting air quality alerts across the region.

As hundreds of firefighters raced to the Angeles National Forest to battle the Colby fire, the South Coast Air Quality Management District warned that air quality was forecast to quickly plummet to unhealthy levels across the region.

They urged residents from the San Gabriel Valley to the Pomona Valley to stay indoors, with the towering plume visible to residents from West Los Angeles to Orange County.

PHOTOS: Colby fire rages near Glendora

But it was in the more immediate area of the fire that the AQMD warned that air quality would be "unhealthy for everyone and possibly worse,” agency spokesman Sam Atwood said.

Smoke and ash from the fire, which exploded to cover more than 1,700 acres in a matter of hours Thursday morning, could affect people with lung and heart disease and asthma as far south as Long Beach, Atwood said.

Commuters on the 10 Freeway reported seeing the flames from as far away as West Covina, with cars emerging from the area caked in dust and ash.

Winds were gusting up to 30 mph Thursday morning and were projected to climb to 40 mph by Thursday night, said Scott Sukup of the National Weather Service. With relative humidity expected to remain in the single digits through Friday and temperatures forecast to hover in the 80s and 90s, “it’s not going to get any better” for firefighters Thursday, he added.

Crews will also have to deal with unpredictable winds at lower elevations where cooler air is being pulled up toward the fire, Sukup said.

“The biggest thing, the hardest thing to predict with the immediate area, are what the winds at the lower elevations are going to do,” he said.

Meanwhile, the AQMD is working to get accurate air quality readings from the area, officials said. Two monitors near the fire don’t appear to be in the path of the smoke, Atwood said, but the agency is sending two portable monitors into the area and expects more accurate readings by the afternoon.

If you can see or smell the smoke, stay in doors and avoid vigorous exercise, Atwood said. Close windows and run an air conditioner if you have one. Paper and surgical masks will not protect you from the pollution, he said. 

The AQMD will post air quality alerts online

Meanwhile, Times reporters were tweeting photos of the smoke cloud.