Bay Area air quality from thousands of fireworks explosions on and around July 4 reached dangerously unhealthful levels in some local neighborhoods, according to air-quality data.

Measurements by the Bay Area Air Quality Management District found high levels of fine particulate matter in various communities — small particles of substances that can easily be inhaled and can lodge in the lungs. The district sensors in four regions recorded the highest hourly fine particulate at levels of 2.5 parts per million on the night of July 4 and into early-morning hours of July 5. On an Air Quality Index scale of green, yellow, orange, red, purple and brown, going from clean air to hazardous, East Oakland ranked 199 (red — unhealthy); southern Santa Clara Valley hit 127 (orange — unhealthy for sensitive groups) and Pleasanton reached 124 (orange — unhealthy for sensitive groups).

In comparison, the air quality in the days and hours leading to the fireworks incidents was in the "good" range (green), with numbers in the single digits or teens, according to district data. But those numbers don't tell the whole story. PurpleAir, an online application that measures air quality on a neighborhood level from air sensors set up by ordinary people at individual homes and businesses, found even higher concentrations of particulates than the district.

In cities such as East Palo Alto and Menlo Park, sensors found particulate levels to be "hazardous," according to PurpleAir, which uses the same Air Quality Index standards as the district.

Menlo Park's Hamilton Avenue sensor, located near Bayfront Expressway, reached a whopping 913 on July 3 at about 5:20 p.m. and 769 on July 4 at 5:10 p.m. In East Palo Alto, sensors showed an area of Glen Way in the neighborhood at 389 on July 4 around 9:30 to 10:30 p.m. Any number above 300 ranks as "hazardous" on the Air Quality Index.

East Palo Alto's Lincoln Street reached 206 at 12:20 a.m. on July 5 and Jasmine Way reached 221 at 10:30 p.m. on July 4, both ranking "very unhealthy," according to PurpleAir. On July 4, Ruth Court ranked 192 at 9:30 p.m. and Westside East Palo Alto reached 167 at 10:50 p.m. or "unhealthy."

Redwood City, which overall ranked "good" air quality on the Bay Area Air Quality Management District sensors, showed an "unhealthy" reading on July 4 at 11:10 p.m. on Chestnut Street, rising to a score of 175. Synapse School in North Fair Oaks reached 116 at 10:40 p.m. on July 4, ranking it "unhealthy for sensitive groups."

In Palo Alto, Corina Way reached 115 at 11:50 p.m. on July 4, or "unhealthy for sensitive groups."

Some parts of Mountain View also had high numbers. On July 4 at 5:40 p.m. Plymouth Street ranked 151, "unhealthy" and Monta Loma reached 109 at 10:50 p.m. or "unhealthy for sensitive groups."

What were we breathing?

Fireworks — even those called "safe and sane" — contain dangerous metals, according to air district data. The 2021 Independence Day Air Quality Assessment by the South Coast Air Quality Management District found high levels of several toxic metals in the Los Angeles Basin from fireworks, a benchmark for the kinds of materials that become airborne by exploding fireworks. These included arsenic, barium, chromium, copper, potassium, manganese, titanium and zinc. Potassium nitrate is used as an explosive in firecrackers; barium and copper compounds produce blue and green colors, for example, in mortars and Roman candles.

These hazardous materials weren't only concentrated locally. They were present in fireworks smoke that drifted across the LA Basin, although the levels didn't exceed health-based standards, the study found.

Some of the hazardous metals can also be more prevalent depending on the year. In 2021, arsenic and potassium were found in higher concentrations than in 2020 due to changes in the composition of fireworks, the study noted.

The Bay Area Air Quality Management District noted that exploding fireworks result in elevated levels of particulate matter (and toxic metals) close to the ground, making it difficult for people to breathe. The smoke and irritating particulates can trigger asthma attacks, coughing, wheezing and eye irritation. Safe and sane fireworks, such as those purchased at pop-up fireworks stands, contain metal salts used to produce colors, but they also cause excessive smoke. Sensitive groups of people are particularly affected, including children and the elderly, according to the district.

Breathing high levels of fine particulate matter can lead to a wide variety of vascular and respiratory effects, including heart attacks, asthma and decreased lung function, the Los Angeles study also noted.