The first air pollution alert of the year was announced yesterday for the Washington area, ending an unusual spell of relatively pollution-free weather.

The Metropolitan Washington Council of Governments declared the alert at 4 p.m., because of what it termed "very unhealthy" levels of photochemical smog and a forecast by the National Weather Service that stagnant air would hang over the region at least until tonight.

During air pollution alerts, COG warns persons with respiratory, heart and eye ailments to curtail their outdoor activities. In addition, it has urged motorists to form car pools or switch to public transportation, such as subways and buses, in an attempt to reduce auto exhaust fumes, the primary cause to smog.

By 4 p.m., COG officials had recorded air quality index levels ranging from 100 to 110 for photochemical smog at three of its monitoring stations in the District of Columbia, Alexandria and Suitland. Such readings slightly exceed federal clean-air standards, designed to protect health.

Smog, also known as photochemical oxidants consists almost entirely of ozone and is produced by a chemical reaction in the presence of sunlight between hydrocarbons and nitrogen oxides, both of which are hyproducts of auto exhaust. Despite yesterday's excessive smog, COG officials reported that none of the area's other major pollutants had risen above federal safety levels. The other main pollutants are carbon monoxide, sulfur dioxide, nitrogen oxides and particulates, such as dust and soot.