A stagnant air mass hovering over eastern Canada pushed the air quality index up to a "very unhealthy" 130 yesterday and the Metropolitan Washington Council of Governments issued the year's second air pollution alert.
The alert was issued at 1 p.m. and will be in effect at least until that time today, COG research assistant Lucille van Ommering said.
There is a 30 to 40 per cent chance of showers today, and rain could help alleviate the stagnant air conditions. If it does not rain, the bad air may plague the Washington area until late this evening or Sunday, according to COG and the National Weather Service.
Yesterday's temperatures rose to the high 80s and the relative humidity was around 50 per cent most of the day, according to the weather service. Hot and humid weather is forecast for much of the weekend.
The air quality index of 130, measured at 3 p.m. at COG's Bethesda monitoring station, equalled the year's previous high registered May 17. The first air pollution alert was issued by COG May 20 and lasted until May 22, van Ommering said.
The highest reading during the previous alert was 110, but winds in the lower atmosphere were weaker, causing poorer dispersal of pollution. Yesterday five of COG's nine reporting stations had readings of 100 or more by 2 p.m.
Air quality descriptions were changed by COG June 6 to make the index consistent with new federal standards. Readings from 100 to 249, such as yesterday's, were previously called hazardous, but now are rated "very unhealthy."
Readings from 0 to 24 continue to be called good; those from 25 to 49 are still considered fair; those from 50 to 99 - previously divided in steps into poor and unhealthy - are rated unhealthy; those from 250 to 750 continue to be rated as dangerous, and above 750 are still rated as very dangerous.
The highest reading recorded here was 180, on Aug. 1, 1975.