SACRAMENTO - After reviewing new medical research about the potential health threat of urban smog, the Air Resources Board (ARB) today required health advisories, commonly known as "smog alerts" to be called at lower concentrations.

The move is designed to provide better health protection for people who are particularly vulnerable to smog-induced illnesses, and especially targets young children and healthy adults who exercise vigorously during high pollution levels.

The ARB decision required health advisories to be issued by local pollution officials when one-hour levels of ozone, or smog, reach .15 parts per million of air (ppm), compared to the old standard of .20 ppm. The advisories suggest postponing or rescheduling heavy physical exercise and other precautions to prevent immediate health problems when smog reaches these higher concentrations, which are almost double the state's long-term health standard of .09 ppm.

The decision is expected to have its greatest effect in the highly polluted South Coast Air Basin, the four-county area in and around Los Angeles, where smog alerts are already issued more than 60 times during the summer ozone season. The area could see as many as 127 health warnings a year under the new limits, based on pollution trends over the last three years.

Currently in that area, schools are urged to postpone heavy physical exercise and athletic team practices until smog levels ease, while other sensitive people are advised to avoid the pollution by remaining indoors and, where appropriate, consult a physician.

Also in that area, some employers voluntarily notify employees to enact previously prepared ridesharing programs to reduce rush hour commute traffic when smog alerts are forecast for the following day.

The new standards, however are also expected to result in health advisories being issued in other metropolitan regions for the first time, including Ventura, southern Santa Barbara, and San Diego counties, which could issue as many as 19 advisories a year under the revised levels.

In addition, health warnings could be issued an estimated 9 - 14 times a year in the Sacramento and San Joaquin valleys, triggered by the lower limits.

In a public hearing, the ARB outlined up to 14 new medical studies conducted in the 1980's that document health damage among very vulnerable people at lower smog levels than previously believed. That evidence builds on other research to outline the sensitivity of groups long targeted by the smog alert, such as young children, the elderly and people with heart and lung disorders.

But the studies also shed new light on the potential health problems among vigorous adults who exercise strenuously, inhaling large amounts of pollution at the same time. The results, according to the studies, can be a pronounced shortness of breath, as ozone constricts the airways, forcing the body to work harder in order to provide sufficient oxygen.

"Not everyone responds to the same amount of pollution in the same way," noted ARB chairwoman Jananne Sharpless during the hearing. "What can be a minor irritation at best for one person can be a serious health threat for others.

 "The increased number of health warnings does not mean that smog is getting worse, but that it causes health problems for some people at levels lower than we previously believed and those people need the added protection that an earlier warning would provide."