It's another sweltering day in the Washington area, with high heat values and poor air quality. And the best thing to do, experts say, is to stay in air-conditioning if possible during the hottest part of this afternoon and evening.

The National Weather Service issued a heat advisory for the District, northern Virginia and suburban Maryland until 8 p.m. Sunday night. The advisory cautioned that heat index values -- temperature and humidity combined -- will hover between 100 and 105 degrees for several hours late this afternoon and early evening.

Air quality will be poor, particularly during the hottest part of the day. People with respiratory illnesses are strongly advised to stay indoors.

Heat index values are also predicted to top 100 degrees in Baltimore, Annapolis and out to the Shenandoah Valley and the counties east of the Blue Ridge in Virginia, the weather service said.

There's a 30% chance of pop-up precipitation around the area this afternoon and evening, the weather service said. And they're predicting the hot hazy weather will last through Monday with another scorching day predicted for the start of the work week.

It's not the afternoon to ride your bike along the canal, especially if you're not used to it. Or take the dog for a long walk. Heat-related injuries are possible if precautions aren't followed. The weather service advised people to reduce outside activity, especially during the hottest part of the day, and to stay out of the sun. It also said to drink plenty of fluids, water being the best.

"Children or pets must never be left alone in cars," the advisory warned, saying temperatures inside a car with the windows rolled up can quickly climb to more than 150 degrees, causing severe injuries and even death.

The elderly and the very young are the most susceptible to heat-related injuries.