RATING FOR AIR QUALITY ADDED.

Byline: Robert Monroe Staff Writer

A new, more stringent index of air pollution goes into effect Tuesday, just in time for the summer smog season.

While the Air Quality Index has the potential to make daily smog reports seem more dire, air quality officials said smog forecasts will be more accurate and helpful for those with respiratory illnesses.

Under existing standards, ``technically you could have situations where air quality is in the unhealthy category but you could be in compliance with standards,'' said South Coast Air Quality Management District meteorologist Joe Cassmassi. The new index is ``giving a little bit of a cushion, making sure those folks susceptible to lower amounts of pollution are protected.''

The new index, which will replace the Pollution Standard Index, adds a new category called ``unhealthful for sensitive groups'' which will fall between moderate and unhealthful smog levels. The revised standards also include measurements of the fine dust particles that do the most damage to lungs.

Greater Los Angeles, which experienced its first year without a smog alert in 1999, still has a solid chance of maintaining the precedent.

Cassmassi predicts that the tail-end of a La Nina weather pattern, which usually means cool weather locally, will dominate the first part of summer. As temperatures rise, so too do the main types of air pollution such as ozone. When the level of the gas reaches .20 parts per billion, it triggers a Stage 1 smog alert.

``Last year, we had no Stage 1 alerts,'' he said. ``If we have normal conditions this year, we hope to repeat that.''

But cooler weather favors windier, clearer skies and higher readings in the new category of very fine particulate matter. Health officials said the readings of fine particles, four times smaller than those measured before, are crucial to people with weakened respiratory systems.

``Once a person has an asthma attack, things can go downhill very quickly,'' said Steven Oppenheimer, director of the Center for Cancer and Developmental Biology at California State University, Northridge. ``There's no question that the more information available about what's in the air is really of critical importance to people with some of these sensitivities.''

The new air quality measure, one of seven the AQMD uses as a basis for its smog forecasts, most likely will drive days of good air quality into the moderate range. Compared to other measures, ultrafine particle concentration levels need not be very high to reach the federal definition of moderate.

``That will be one of our main triggers to issue more alerts to people,'' Cassmassi said.