Objectives:To determine whether pre-exposure to an acidic fog increases the effects of acute ozone exposure in healthy human subjects.

Findings: Pre-exposure to acidic fog did not have an additive or synergistic effect on ozone exposure responses. However, the results indicated the possible existence of a susceptible or sensitive population subgroup, who appear healthy and/or are asymptomatic, who are adversely affected by ozone exposure.

Importance to ARB's Program: The results were used in the final assessment of the Kapiloff program projects. (The Kapiloff program was the first formal investigation of atmospheric acidity in the world. The ARB's Atmospheric Acidity Protection Program was a more detailed investigation that was implemented as a result of Kapiloff findings.) If the increased sensitivity in the silent or asymptomatic group is confirmed in a randomly selected group from the general population, it will be necessary to modify the health advisories issued to protect the sub-group.