Sulfur dioxide (SO2) is the gas you smell faintly when a match strikes. Industrially, it forms whenever you burn fuel that contains sulfur, historically coal, today still some heavy oil, diesel, and the unrefined "bunker" fuel used by ocean shipping. It is also a major component of volcanic emissions and a minor component of wildfire smoke.
Acute health effects are concentrated in the airways. EPA's primary NAAQS is 75 ppb over a 1-hour average, set to protect asthmatics, who can experience bronchoconstriction at exposures below the limit. WHO's 2021 guideline is tighter: 40 µg/m³ as a 24-hour average.
Terrestream does not measure SO2 directly: the SEN66's MOX gas array doesn't differentiate sulfur compounds from other reducing gases. Instead, the dashboard pulls outdoor SO2 from the Google Air Quality API. The number is a regional estimate, not a hyperlocal reading.
For most users in residential areas, outdoor SO2 is now low; coal-plant retirements and low-sulfur diesel mandates have driven 20+ years of steady declines. Spikes typically come from nearby industry, port operations (cruise and container ships at idle), or volcanic events (Hawaii, Iceland, Indonesia). If your area surfaces an elevated SO2 reading, the dashboard treats it like any other outdoor gas notification: suppress ventilation, run filtration, watch for asthma symptoms.
References
- EPA - Sulfur dioxide basics www.epa.gov
- WHO Global Air Quality Guidelines (2021) www.who.int
- Open-Meteo - Air Quality API documentation open-meteo.com
- Copernicus CAMS - Global atmosphere monitoring atmosphere.copernicus.eu
