Ambient air pollution levels vary seasonally, day-to-day, and
by time-of-day. For example, ultraviolet light from the sun
activates the chemical reactions that form ozone, generally
leading to higher concentrations in late morning through
early evening (25). Alternatively, ozone concentrations may
peak later in the evening or at night in locations that are
downwind of ozone formation (25). Levels of air pollutants
also vary in different microenvironments, such as outdoors
in variable proximity to sources, at home, at workplaces,
in schools, in vehicles, etc. Individuals can know when air
pollution levels are likely to be elevated either by sensing
poor air quality (odor, irritation, symptoms), having
knowledge of conditions that tend to lead to higher air
pollutant levels in their area, or via public communications
based on measured or predicted levels at air monitoring
stations. In order to most effectively adjust behavior to
reduce exposure and risk, individuals must be able to
anticipate when and where air pollutant levels are likely to
be elevated above levels thought to confer increased risk.