Crews battling large hay fire in town of Laveen

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LAVEEN, Ariz. (AP) -- Crews are battling a large hay fire in the town of Laveen, west of Phoenix.

Phoenix Fire Department officials say the fire was reported just south of 67th Avenue and Baseline Road around 4:45 p.m. Wednesday.

It's still unclear how the fire started.

Authorities say the haystack burning is 200-feet by 50-feet by 20-feet high.

It's about 1,500 feet away from a structure, but there's no report of any structural damage so far.

Phoenix Fire Capt. Scott McDonald says crews have secured water supplies and surrounded the haystack from all sides and they're working to extinguish the flames.

State Department of Environmental Quality officials also are at the scene to monitor air quality from all the smoke in the area of the fire.

People quite a distance from the fire reported seeing and smelling the smoke. Residents of nearby neighborhoods have been told to stay inside and keep their doors and windows closed.

While the cause of the fire has not been determined, it's possible that spontaneous combustion is to blame.

Plant-cell respiration generates heat. If the bales are too moist, mesophilic (warm temperature) bacteria can grow, causing the interior temperature of a hale bale to reach between 130 and 140 degrees. Normally the bacteria dies and the interior temperature eventually stabilizes.

Sometimes, however, thermophilic (heat loving) bacteria comes into play, raising the interior temperature of the bales even more and damaging the hay itself. Once a bale's temperature hits 175, a fire is usually imminent.

"This damaged material combines readily with oxygen at high temperatures and can self ignite in the presence of oxygen," to the Virginia Cooperative Extension at Virginia Tech.

Because the source of the heat is so deep inside haystacks, such fires can be very difficult to extinguish. And once the fire is out, there's a danger of flames sparking back up.