Kidsdata.org offers two measures of outdoor air quality related to criteria air pollutants:

• The annual average concentration of fine particulate matter (PM2.5) in the air
• The number of days per year with unhealthy ground-level ozone concentrations (i.e., above the national standard of 0.070 parts per million)

Depending on the indicator, data are available for counties, the state (as averages across counties), and/or the nation (as averages across testing sites).

Air pollution is a serious threat to children's health, with links to adverse birth outcomes, obesity, cardiovascular and respiratory diseases, and cancer (1, 2). Long-term effects also can extend beyond physical health—exposure to contaminants, especially at high concentrations and durations, is associated with deficits in cognitive and behavioral development (1, 2). Children are more vulnerable to air pollution than adults, and younger children more vulnerable than older children, because their bodies and organs are less fully developed, and they breathe more air relative to their size, resulting in greater proportionate exposure (1, 2, 3). They also may experience greater exposure due to childhood activities like outdoor play (3).

Air pollution can occur outdoors or indoors. Motor vehicles and industrial facilities are major sources of common outdoor air pollutants, such as ground-level ozone (a main component of smog), fine particulate matter, lead, and noxious gases (1, 4). These pollutants can flow indoors, where the air may be further polluted by fuel-burning stoves and appliances, household chemicals, building materials, and tobacco smoke (1, 5). Among common outdoor air pollutants, fine particulate matter and ground-level ozone are considered the greatest threats to human health (4).

In 2019, the average air concentration of fine particulate matter (PM2.5) across California counties was 8.1 micrograms per cubic meter (µg/m³), a drop of more than 50% compared with 1999 (17.6 µg/m³). This decrease at the state level echoes national trends. At the county level, six of the 44 counties with data in 2019—all in Southern California and the Central Valley—recorded average PM2.5 concentrations above the national public health standard of 12 µg/m³.

Of the 49 California counties with ground-level ozone data in 2019, 15 did not record any days when ozone concentrations exceeded the current national regulatory standard of 0.070 parts per million (ppm), whereas six recorded more than 30 such days. Forty years earlier, 17 counties recorded 30 or more days above 0.070 ppm.

Air pollution is a critical public health issue (1). Children, due to their developing bodies and greater air intake relative to body weight, are more vulnerable to air pollutants than adults (1, 2). Exposure to pollutants in the womb, in the home, or in the community may cause serious, lasting health problems and limit children's physical and cognitive development, academic performance, and economic productivity in adulthood (1, 3).

California is considered a leader in air quality policy, with strong regulations addressing vehicle and industrial emissions, secondhand smoke, building practices, and other contributors to outdoor and indoor air pollution (4, 5, 6). Still, the state is home to many of the most polluted cities in the nation, with low-income communities and people of color at higher risk of exposure to air pollution than other groups (6, 7). While significant progress has been made, much more work is needed to ensure that all California children consistently have clean air to breathe and healthy environments in which to grow and thrive (7, 8).

Policy and program options to improve air quality, and children's environmental health overall, include continuing to:

• Enforce and strengthen laws and regulations limiting the generation of vehicle emissions and other contaminants, along with agricultural and industrial practices that cause air pollution and pesticide exposure; as part of this, maximizing community-based action to help address local pollution burdens, in accordance with state law (4, 8)
• Promote effective strategies to decrease exposure to wildfire smoke in vulnerable communities, such as providing cleaner air shelters and public education on wildfire preparedness and appropriate use of air conditioners and filters (9)
• Enforce and expand local and state policies to reduce secondhand smoke exposure, particularly in multi-unit housing settings (5)
• Improve and implement policies and guidelines that improve indoor air quality in homes, schools, and child care settings, addressing issues such as ventilation, mold, pests, hazardous building materials, chemical exposure, and outdoor sources of pollutants (6, 10, 11)
• Support public education and professional workforce training in multiple sectors (e.g., housing, child care, health care, etc.) on the risks of unhealthy indoor environments and how to make improvements (10, 11)
• Encourage a broad focus on multiple environmental hazards—e.g., advancing strategic partnerships among state and local organizations focused on health, housing, education, land use planning, transportation, and environmental protection—rather than focusing on single sectors or contaminants (8, 10, 11)
• Increase overall awareness among health care professionals, policymakers, advocates, and others regarding children's environmental health, including the effects of contaminants on children, as well as the disproportionate burdens faced by some communities (7, 8)
• Support additional research on the connections between the environment and children's health, and strategies to prevent and address adverse effects; also, maintaining and strengthening environmental health data systems to continue closing information gaps (8)