Washington area residents returning to work yesterday after the long July 4 weekend discovered another advantage of the new Metro subway system. On hot, muggy days, it's one of the coolest places in town.

The sleek, silver trains provided riders with air-conditioned comfort while area residents who remained aboveground sweltered through the 11th consecutive day of higher than 90 degree temperatures here.

The National Weather Service reported that the themometer reached 98 degrees at National Airport between 4 and 5 p.m. yesterday, highest temperature recorded this year.

"With the humidity above ground, its kind of hard for me to breath," said Robert Van Hoeson as he rushed to catch a subway train at the McPherson Square Metro station. "Down here, the air is a lot fresher and it seems a lot cooler," he said.

The Metropolitian Council of Governments (COG) issued an air pollution alert at 3 p.m. after three measuring stations in the metropolitan area showed readings of 100 or over. Dennis Bates, director of health and environmental protection at COG, said readings of 100 to 250 indicate "very unhealthy" air quality.

Bates urged citizens with respiratory problems to reduce their level of activity and urged the general public to reduce driving as much as possible during the alert.

A Weather Service forecast calls for continued hot and humid weather today and Thursday with a chance of afternoon and evening thundershowers both days.

But for some people, such as D.C. Police Officer L.C. Gooch, things can always get worse. As he fought off pigeons from the water fountain at McPherson Square NW, Gooch remarked: "You think this is hot, just wait 'til August comes."