With another hot and humid day on Long Island, officials are urging residents to take common-sense precautions against heat-related ailments and to conserve power to avoid overextending the electrical grid.

"I encourage everyone to remain indoors when possible, stay hydrated, and check on neighbors who may need assistance," Gov. Andrew M. Cuomo said in a statement. "Together, we can make sure that everyone stays safe during this period of extreme heat."

An increase in ground-level ozone will trigger an air quality health advisory from 11 a.m. to 11 p.m. Wednesday, and the state Department of Environmental Conservation is warning people working outdoors to consider limiting any strenuous physical activity.

High temperatures are likely to be from near 90 to the mid-90s on Long Island, but lower on the Twin Forks, the National Weather Service said.

State officials said New Yorkers should drink two to four glasses of water per hour during extreme heat; limit physical exercise to the early morning or late evening when temperatures are lower; stay out of the sun if possible; and seek out air-conditioned spaces when possible.

People should also check on elderly neighbors, and should never leave pets or children unattended in parked vehicles.

Nassau County and the Town of Hempstead said community and senior centers would be available as cooling centers during daytime hours.

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Through Friday, 15 cooling centers will be open in Hempstead, where there also will be extended hours at 15 pools and eight beaches.

The Town of North Hempstead on Thursday afternoon opened a cooling center at Michael J. Tully Aquatic Activities Center in New Hyde Park. Officials said cold water will be provided at the air conditioned center, and it is staying open until 9 p.m.

Islip Town Supervisor Angie Carpenter encouraged seniors in particular to seek relief from the heat by visiting air-conditioned senior centers in Brentwood, East Islip, Central Islip, Oakdale, Ronkonkoma and West Islip from 8:30 a.m. to 4:30 p.m.

The heat wave also endangers dogs, cats, horses and farm animals, and Suffolk's SPCA is investigating two calls reporting horses that could be in trouble, left out in the direct sunlight, for example, without clean water or shade, said Chief Roy Gross.

The SPCA sent out an advisory on safeguarding pets during hot summer weather, noting that even horses can get sunburn: http://www.horses-and-horse-information.com/articles/0398hot.shtml

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PSEG Long Island said it has enough capacity to handle the added demand, but asked customers to help by following energy conservation measures:

Eliminate nonessential electric consumption.

Put air conditioners on timers and don't let them run when not at home.

Set air conditioners at 78 degrees or higher if health allows.

Use fans to circulate cool air, which helps cut air conditioner use.

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Set refrigerators and freezers at most efficient temperatures.

Run major appliances such as washers, dryers, dishwashers, and pool pumps in the morning or late evening to avoid peak demand hours of 2 p.m. to 8 p.m.

The New York Power Authority and the MTA said they will activate a program to reduce electricity use at times of high demand, but that mass transit service will not be affected.

The MTA said it would lower electricity use at 28 New York City Transit bus depots, all MTA NYC Transit substations, and one rail service yard.

The program was activated Wednesday for the first time this summer, and NYPA will pay the MTA $25 per kilowatt reduced.

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With Joan Gralla, Scott Eidler and Sophia Chang