Sulfur dioxide (SO2) is overwhelmingly outdoor-driven. The major sources globally are coal-fired electricity generation (declining in North America and Europe, still significant in much of Asia), maritime shipping near major ports (heavy fuel-oil bunker combustion), oil refining, and volcanic emissions during eruptions. Indoor sources are essentially nil except for kerosene heaters and a few specialty industrial processes; residents in homes near these sources see indoor SO2 tracking outdoor SO2 with a damping factor similar to ozone (0.3-0.5 in closed homes).
The playbook overlaps PM2.5. SO2 chemistry in the atmosphere converts much of it to ammonium sulfate aerosols (PM2.5) within hours to days. For health-effects purposes, the SO2-driven exposure is often dominated by the secondary PM2.5, not the SO2 itself. The interventions for reducing PM2.5 apply directly: MERV-13 HVAC filtration, portable HEPA per CADR, and envelope sealing during alerts.
SO2-specific notes. Activated-carbon filtration removes SO2 reasonably well, faster than ozone, slower than VOCs; whole-house carbon filters at HVAC return are worth considering for residents within several miles of a coal plant, port, or refinery. For volcanic-emission events (Hawaii, Iceland, parts of Italy and Indonesia), close windows during the highest VOG ("volcanic smog") levels and run HVAC on recirculate with a fresh carbon filter; these events can last days to weeks.
Long-term remediation. The most impactful intervention is usually political rather than technical: support local air-quality regulation, industrial emission standards, and port-electrification efforts. From a household perspective, knowing your exposure profile is the first step; the dashboard's outdoor SO2 from the Google Air Quality API is more reliable than most consumer SO2 sensors. See outdoor air feeds for the data source and sulfur dioxide for health-effects background.
This is general guidance, not a substitute for professional assessment of your specific home. Major interventions (HVAC redesign, sealing a leaky envelope, mold remediation, electrical work for fans or venting) should be done with a certified professional. For chronic problems that don't respond to the steps here, see when to call a pro.
References
- EPA - Sulfur dioxide basics www.epa.gov
- WHO - Global air quality guidelines (SO₂) www.who.int
- USGS - Volcanic smog (vog) FAQ www.usgs.gov
- Copernicus CAMS - Global atmosphere monitoring atmosphere.copernicus.eu
