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 with data), and/or the nation (as averages across testing sites).

Air pollution is a serious threat to children, with causal links to negative health impacts over the life course including adverse birth outcomes, impaired growth and development, obesity, cardiovascular and respiratory diseases, stroke, cancer, and disorders of the neurological, immune, and reproductive systems (1, 2, 3). Long-term effects also can extend beyond physical health—exposure to contaminants, especially at high concentrations and durations, is associated with cognitive, mental, and behavioral problems (4, 5). Even at low levels of exposure, air pollution may harm children's health and growth, particularly during the prenatal, early childhood, and early adolescent stages of development (2, 3, 4, 6). Children are more vulnerable to air pollution than adults because they breathe more air relative to their size, their bodies and organs are less fully developed, and they generally are more active and spend more time outdoors—resulting in greater proportionate exposure (3, 4, 6).

Air pollution can occur outdoors or indoors. Motor vehicles, power plants, industrial facilities, and wildfires are major sources of common outdoor air pollutants, such as ground-level ozone (a main component of smog), fine particulate matter, and noxious gases (2, 3, 7). These pollutants can flow indoors, where the air may be further polluted by appliances (fuel-burning or electric), emissions from cooking, smoke, mold, and chemical vapors from household products (1). Research has identified air pollution, especially fine particulate matter, as one of the greatest global threats to human health (6, 8).

Children of color and those with lower household income are more likely than their peers to be exposed to air pollution and experience its related health impacts, as are children living in neighborhoods near industrial facilities or major highways, in agricultural areas, or in substandard housing (1, 2, 9). The increased risk of pollutant exposure for these children may lead to lower school achievement and attendance due to asthma and other health issues (1, 2).

In 2023, 17 California counties recorded an annual average airborne concentration of fine particulate matter (PM2.5) above the current national public health standard of 9 micrograms per cubic meter—down from 39 counties in 2020. A majority of counties in Southern California and the San Joaquin Valley averaged PM2.5 concentrations above the national standard in 2023.

Of the 49 California counties with ground-level ozone data in 2023, 19 did not record any days when ozone concentrations exceeded the national regulatory standard of 0.070 parts per million, whereas four (Kern, Los Angeles, Riverside, and San Bernardino) recorded 52 or more such days—the equivalent of at least one unhealthy day each week.

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 (2). Exposure to pollutants in the womb, in the home, in schools, 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 a leader in policies that promote air quality and children's environmental health, with strong regulations and practices addressing contributors to outdoor and indoor air pollution (4, 5). Still, the state is home to many of the most polluted areas in the nation, with low-income communities and people of color at higher risk of exposure to air pollution than other groups (3, 5, 6, 7). The state also faces increasing air quality challenges related to climate change and extreme wildfires. While significant progress has been made in recent decades, 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 (5, 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 (4, 5, 8)
• Support policies and regulations that increase active transit and public transportation systems, reduce urban sprawl, and expand local zoning to allow development of affordable housing, jobs, and services close to public transit (9)
• Maximize community-based action to help address local pollution—for instance, promoting "smart surfaces" such as solar panels, green roofs, and reflective pavements, which mitigate heat and improve air quality (5, 7, 8)
• Promote the use of cleaner air shelters and other strategies to decrease exposure to wildfire smoke in vulnerable communities, and educate diverse audiences regarding best practices to protect children during and after wildfires, e.g., by using high-efficiency portable air cleaners (5, 10)
• Enforce and expand local and state policies to reduce secondhand smoke exposure, particularly in multi-unit housing settings (1)
• Strengthen 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, household chemical exposure, and outdoor sources of pollutants (1, 5)
• Support public education and professional training in multiple sectors on the risks of unhealthy indoor environments and how to make improvements (1, 5)
• Elevate awareness of children's environmental health among health care professionals, policymakers, advocates, and others, including the effects of contaminant exposure and the disproportionate burdens faced by some communities (1, 3, 5)
• 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 (1, 5)
• Support 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 (1, 5)
• Maintain progress on federal clean air regulations, and preserve adequate funding and staffing levels for the Environmental Protection Agency (7)