Starting Monday, an air quality index reading, above 100 will no longer be hazardous to your health. It will be very unhealthy.

The Metropolitan Washington Council of Governments, which has been publishing an index since 1973 based on readings at its netweok of air-monitoring stations, will make changes in its descriptions of air quality.

The indexes and descriptions are regular parts of weather reports published in area newspapers and broadcast by radio and television stations.

They are the basis of periodic pollution alerts.

Dennis R. Bates, COG director of health and environmental protection, said the index itself - the numbers reported for the thrice daily readings - will not be changed. Those numbers are based on measurements of various pollutants, predominantly those created by automobiles.

Bates said the new descriptive phrases will make the COG index consistent with new federal standards.

Under the new plan, readings from 0 to 24 will continue to be called good; those from 25 to 49 will continue to be fair; those from 50 to 99 will become unhealthy (previously divided in steps into poor and unhealthy): those from 100 to 249, previously rated hazardous, will be rated as very unhealthy; those from 250 to 750 will continue to be rated as dangerous and above 750 will continue to be rated as very dangerous.

The highest index reading here was 180, recorded on Aug. 1, 1975.