Smog (ozone + NO2)

AIR

~10xfederal NO2 limit vs. health guideline

The federal annual standard for nitrogen dioxide, set in 1971 and unchanged since, sits roughly ten times above the WHO 2021 health guideline. The federal ozone standard is more than double the WHO peak-season level.

Settled science

Stasis Home ResearchEvery claim on this page is graded against peer-reviewed health research and linked to its primary source.Updated 2026-06

Smog is ground-level ozone plus nitrogen dioxide, formed when traffic and combustion exhaust cook in sunlight. The air you can see from the freeway is the air that infiltrates your home. Chronic exposure raises respiratory and cardiovascular mortality, with no safe threshold. The route Stasis can change is your indoor air: carbon filtration, envelope sealing, and forecast-driven window timing.

What it is, and where it comes from

Smog is two pollutants that travel together. Ground-level ozone (O3) is not emitted directly. It forms when nitrogen oxides from vehicle exhaust react with volatile organic compounds under sunlight, which is why it peaks on hot, traffic-heavy afternoons. This is the brown layer over the freeway, distinct from the protective ozone miles up in the stratosphere. Nitrogen dioxide (NO2) is one of the precursors and a pollutant in its own right, coming mostly from cars, diesel, refineries, and ports outdoors, and from gas cooking, attached garages, and unsealed gas appliances indoors. Both ride in from regional outdoor air. Outdoor exposure dominates for most households, infiltrating buildings at roughly one part in five to two parts in three of the outdoor level depending on how tight your envelope is. Indoor gas combustion adds to it during use, but for most homes the outdoor air sets the daily dose.

Why it matters

Ozone and nitrogen dioxide are oxidizing gases. They react with the lining fluid of your airways, generating reactive oxygen species that inflame and damage the cells lining your lungs, with downstream effects on the cardiovascular system through arterial stiffness and a more clot-prone blood state. The mortality evidence comes from large multi-decade cohorts, not models. In a study following more than 669,000 adults for 22 years, each step up in long-term ozone tracked with higher respiratory mortality, the strongest signal, alongside smaller increases in all-cause and circulatory death. For nitrogen dioxide, pooled long-term studies link each increment to higher cardiovascular and respiratory mortality. The position both the EPA and WHO hold is that there is no level below which the risk drops to zero. Children take the hardest hit: their lungs are still developing, they spend more time outdoors during peak-ozone hours, and gas-stove exhaust indoors is tied to a meaningful share of childhood asthma.

In a 22-year study of more than 669,000 adults, each 10 ppb step up in long-term ozone exposure was associated with about 12 percent higher respiratory mortality, the strongest endpoint, plus smaller increases in all-cause and circulatory death.Turner et al., 2016 (Am J Respir Crit Care Med)
Pooled long-term studies link each 10 µg/m3 increment of nitrogen dioxide to roughly 13 percent higher cardiovascular mortality and a few percent higher all-cause mortality.Faustini, Rapp, Forastiere, 2014 (Eur Respir J)
About 12.7 percent of current US childhood asthma is attributable to gas-stove use, and in California the share is 20.1 percent, comparable to the burden from secondhand smoke.Gruenwald et al., 2023 (Int J Environ Res Public Health)
In the South Coast Air Basin around Los Angeles, the canonical photochemical smog region, the 2021-2023 ozone design value is 106 ppb, 51 percent above the 70 ppb federal standard, and the basin is in EXTREME nonattainment with a 2037 deadline it is not on track to meet.US EPA Federal Register, Aug 2024: South Coast ozone design value 106 ppb

What we grade it against

Where the health research draws the line, versus the legal limit.
Contaminant	Health-based level	Legal limit	Source (health-based)
Ozone, 8-hour (ppb)	~30.6WHO peak-season guideline, no-threshold position	70EPA standard, 2015 revision	WHO Global Air Quality Guidelines, 2021
Nitrogen dioxide, annual (µg/m3)	10WHO 2021 guideline, tightened 4x on mortality evidence	~99.6EPA 53 ppb annual, set 1971, unchanged	WHO Global Air Quality Guidelines, 2021
Ozone, 8-hour, California (ppb)	~30.6WHO health-based goal	70CARB state standard, matches federal	CARB California Ambient Air Quality Standards
Example: South Coast basin, CA, ozone (ppb)	~30.6WHO health-based goal	1062021-2023 measured value in the worst-case US basin, EXTREME nonattainment	US EPA Federal Register, Aug 2024

Health-based levels come from peer-reviewed research and government risk scientists working without cost constraints. Legal limits are enforceable compromises. Your report grades to the health column.

What helps

Direct fixes

Standalone HEPA plus activated-carbon air purifier (bedroom and main living area)
A properly sized unit with a real carbon stage of several pounds, not a thin coated mesh, delivers roughly 40 to 70 percent ozone reduction and 20 to 40 percent nitrogen dioxide reduction in the room it serves. This is where the gas-phase work happens.
HEPA alone removes particles, not gas-phase ozone or NO2. The carbon stage does the gas work, and a 'HEPA purifier' sold without substantial carbon is a particle product. Carbon is moderate, not magic: it buffers the regional air, it does not reach zero.

Bigger retrofits

Whole-house MERV-13 plus activated-carbon HVAC filter
Moderate gas-phase reduction for outdoor ozone and NO2 infiltrating through the fresh-air intake and envelope leaks, with dual benefit against PM2.5. The lowest-friction option to install on systems built since 2000, after a static-pressure check on older equipment.
A standard one-inch slot holds only a few pounds of carbon, so gas-phase performance is partial. Higher-capacity carbon banks reach true gas-phase performance but run into four-figure install and recharge territory.
Building envelope tightening
Weather stripping, door sweeps, recessed-light gaskets, and attic and damper sealing cut infiltration of outdoor ozone, NO2, and PM2.5 together, with a secondary energy-cost benefit.
Sealing reduces how much outdoor smog gets in, but does nothing about indoor combustion sources already inside, such as a gas range.

Free and behavioral

Forecast-driven window and HVAC protocol (Stasis app)
Real-time and 24-hour regulatory-monitor forecasts drive automated nudges: close windows and switch HVAC to recirculate on high-ozone afternoons, flush with outdoor air in the cleaner morning and evening, run the bedroom purifier on high during smoke events.
Behavioral timing reduces dose, it does not filter the air. It works alongside the carbon and sealing, not instead of them.
Induction conversion and sealed-combustion appliances at replacement time
If a kitchen renovation is already in play, an induction range removes the indoor NO2 combustion source. Choosing sealed-combustion gas water heaters and furnaces at replacement eliminates indoor bleed at near-zero incremental cost.
Not worth a standalone renovation for air quality alone. The filtration and HVAC work do most of the lift for most households.

What is and is not being fixed for you is worth saying plainly. The federal ozone standard sits more than double the WHO health-based level, the federal annual NO2 standard has not moved since 1971, and a large share of the US population lives in counties that fail to meet even the federal ozone standard. In the worst US case, the South Coast basin around Los Angeles, the attainment deadline runs to 2037 on a plan the region is not on track to meet. The regulators are working a feasibility problem on a multi-decade clock. Your children are not on that clock. The same outdoor air gets into your home, and the science that it harms is settled. Waiting for the air outside to be fixed is not a plan. Protect the air you can control inside, with carbon filtration, a tighter envelope, and smarter window timing, because that is the part you can change now.

Common questions

Does a HEPA air purifier remove smog?

Only the particle half. HEPA captures PM2.5 but does nothing for gas-phase ozone or nitrogen dioxide, which are the smog gases. The gas work is done by a substantial activated-carbon stage, several pounds of it, not a thin carbon-coated mesh. Read the spec sheet for grams of carbon, not just the word HEPA. A well-sized carbon unit reduces ozone in a room by roughly 40 to 70 percent and NO2 by 20 to 40 percent.

If the windows are closed, is the smog outside really my problem?

Yes, because outdoor air still infiltrates. Even with windows shut, ozone and NO2 enter through envelope leaks and the HVAC fresh-air intake, reaching roughly one part in five to two parts in three of the outdoor level depending on how tight your home is. A tighter envelope lowers that fraction, which is why sealing is part of the answer alongside filtration.

Should I get an ozone generator or an ionizer to clean the air?

No. California banned ozone generators for residential use because they emit ozone as their operating mechanism, which is the pollutant you are trying to avoid. Bipolar ionizers and PCO devices have not shown reliable residential efficacy and some emit secondary pollutants. The proven path is HEPA for particles, activated carbon for gases, envelope sealing, and forecast-driven timing.

I have a gas stove. How much does that matter versus the air outside?

For most homes the outdoor air sets the daily dose, and indoor gas cooking adds to it during use. It still matters: gas-stove use is tied to about 12.7 percent of US childhood asthma, and 20.1 percent in California. Venting the range hood outside during cooking helps, and if a kitchen renovation is already planned, induction removes the indoor source entirely.

Why is the air still bad after decades of regulation?

The standards are set to what regional industry and vehicle fleets can feasibly achieve, not to the level the science says is safe. The federal ozone standard is more than double the WHO health-based level, and in the worst US case, the South Coast basin around Los Angeles, measured ozone runs 51 percent above even the federal number on an attainment timeline that stretches to 2037. That gap is the exposure you can buffer at home.