A blog post from an independent researcher has quietly surfaced one of the more fascinating personal datasets in climate and indoor air quality science — and the implications reach far beyond one person’s apartment. Cap’n Transit, the pseudonymous urbanist and transit advocate behind Grieve-Smith.com, recently published nine observations drawn from years of personal carbon dioxide monitoring using consumer-grade sensors. The findings are granular, honest, and occasionally unsettling. They also happen to confirm, complicate, or challenge several mainstream assumptions about indoor air quality, ventilation, and the post-pandemic fixation on CO₂ as a proxy for health risk.
The setup is deceptively simple. A CO₂ monitor — the kind now sold by companies like Aranet and Vitalight for under $200 — sits in a living space and logs readings continuously. Outdoor CO₂ concentrations hover around 420 parts per million these days, a number that itself tells a grim story about fossil fuel emissions. Indoors, the number climbs. How fast and how high depends on room size, occupancy, ventilation, and whether anyone remembered to crack a window.
Here’s what makes the Grieve-Smith data interesting: it’s not from a lab. It’s from real life.
The first observation is one that ventilation engineers know well but that the general public still underestimates. CO₂ levels in a closed bedroom with one or two sleeping adults rise shockingly fast. Within hours, readings can exceed 2,000 ppm — roughly five times the outdoor baseline. At those levels, research has linked elevated CO₂ to impaired cognitive function, disrupted sleep quality, and next-day grogginess that people routinely attribute to poor rest rather than poor air. The author notes that simply opening a window or running a fan with access to outside air can cut these readings dramatically. But in winter, in a city apartment, that’s not always practical. And so millions of people wake up every morning having marinated in their own exhalations for eight hours, none the wiser.
This tracks with a growing body of peer-reviewed literature. A 2021 study published in Environmental Health Perspectives found that residential bedroom CO₂ concentrations frequently exceeded 1,000 ppm overnight, the threshold above which the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommends improved ventilation. A 2015 study from the Harvard T.H. Chan School of Public Health — the now-famous COGfx study — demonstrated measurable cognitive performance declines at 950 ppm and above. The bedroom, it turns out, is one of the most poorly ventilated rooms most people occupy, and they spend a third of their lives in it.
The second observation from the monitoring data concerns cooking. Gas stoves, already under regulatory and public health scrutiny, are significant CO₂ emitters indoors — on top of the nitrogen dioxide, particulate matter, and methane they release. The Grieve-Smith data shows CO₂ spikes during cooking that rival or exceed overnight bedroom accumulation, but over much shorter timeframes. The concentration climbs fast, peaks, and then dissipates depending on whether a range hood vents to the outside or merely recirculates air through a charcoal filter. Many apartment range hoods do the latter. They’re essentially decorative.
This dovetails with reporting from The New York Times, Vox, and others over the past two years on the indoor air quality consequences of gas cooking. The Consumer Product Safety Commission considered and then backed away from regulatory action on gas stoves in 2023, but state and local governments — particularly in New York and California — have moved forward with electrification mandates for new construction. The CO₂ data adds another dimension to an argument that has largely focused on NO₂ and childhood asthma risk.
Observation three: occupancy matters more than almost anything else. A room with four people in it will see CO₂ climb four times faster than a room with one, all else being equal. This is elementary physiology — each person exhales roughly 200 milliliters of CO₂ per minute at rest, more during exertion. But the monitoring data makes the math visceral. Hosting a small dinner party in a modest apartment can push readings above 2,500 ppm within an hour or two. That’s the kind of number that, if measured in an office building, would trigger an HVAC investigation.
And yet nobody investigates their dinner party.
The fourth observation is about the pandemic-era enthusiasm for CO₂ monitoring as a proxy for COVID-19 transmission risk. The logic was straightforward: CO₂ is a marker of rebreathed air, and rebreathed air is the vehicle for airborne pathogens. Higher CO₂ means more shared air, which means higher transmission probability. Schools, restaurants, and offices installed monitors. Some jurisdictions mandated them. Belgium required CO₂ displays in public venues. The idea was sound in principle.
But the Grieve-Smith data highlights a subtlety that got lost in the public messaging. CO₂ is a useful but imperfect proxy. It tells you about ventilation relative to occupancy, not about infection risk per se. A room at 800 ppm with one infected person may be riskier than a room at 1,200 ppm with no infected people. The monitors measure air exchange, not pathogen load. This distinction matters for policy. Mandating CO₂ thresholds is a blunt instrument — a helpful one, but blunt. The author doesn’t dismiss the approach. He contextualizes it.
Fifth: outdoor air quality complicates the ventilation equation. Opening windows is the simplest way to reduce indoor CO₂, but in cities with heavy traffic, wildfire smoke, or industrial emissions, the outdoor air you’re inviting in carries its own hazards. The monitoring data shows that on high-pollution days, the tradeoff between CO₂ dilution and particulate matter exposure is real and unresolved. Filtration helps — HEPA filters and Corsi-Rosenthal boxes can clean particulates without raising CO₂ — but they add cost and noise. For low-income households in polluted neighborhoods, the ventilation dilemma is sharpest. Open the window and breathe dirty air, or close it and breathe stale air. Neither option is good.
This is not a new tension, but it’s one that indoor air quality advocates have struggled to communicate clearly. The Environmental Protection Agency’s guidance on indoor air quality still treats ventilation and filtration as complementary strategies, which they are in well-resourced buildings with mechanical ventilation systems. In a fifth-floor walkup in the Bronx with single-pane windows and no central air, the strategies diverge. Policy that ignores this divergence fails the people who need it most.
The sixth observation is more personal. The author notes that CO₂ monitoring changed his behavior. Knowing the numbers made him open windows more often, cook differently, and think about room airflow in ways he hadn’t before. This is the behavioral nudge argument for consumer CO₂ monitors — the same argument that drove the Nest thermostat’s success and the adoption of home energy monitors. Information changes behavior, even without mandates. But it only works if people buy the monitor, understand the readings, and have the means to act on them. That’s a lot of ifs.
Seven: the data reveals seasonal patterns. Winter readings are consistently higher than summer readings, for obvious reasons — windows stay closed, heating systems recirculate indoor air, and people spend more time inside. In cold climates, the indoor CO₂ penalty of winter can be substantial. This has implications for school performance (classrooms in January are measurably stuffier than classrooms in May), workplace productivity, and respiratory illness seasonality that may have nothing to do with the pathogens themselves and everything to do with the ventilation context in which they spread.
Observation eight addresses the sensors themselves. Consumer CO₂ monitors have improved enormously in accuracy and affordability over the past five years, but they’re not all created equal. The author notes that nondispersive infrared (NDIR) sensors — the type used in devices like the Aranet4 — are generally reliable, while cheaper metal oxide sensors can drift significantly and give misleading readings. For anyone considering a purchase, the sensor technology matters. A bad reading is arguably worse than no reading at all, because it breeds false confidence.
The ninth and final observation is the broadest. CO₂ monitoring, the author argues, is a window into a much larger failure of building design and public health infrastructure. We spend approximately 90% of our time indoors, according to EPA estimates, yet indoor air quality remains largely unregulated for residential buildings. Commercial buildings must meet ASHRAE ventilation standards. Homes don’t. The air in your office is governed by code. The air in your bedroom is governed by luck.
This regulatory gap has drawn increasing attention. In early 2025, the White House released a national indoor air quality strategy as part of its broader public health agenda, though its provisions were largely voluntary. ASHRAE updated its Standard 241 in 2023 to address airborne infection risk in buildings, a direct response to the COVID-19 pandemic, but adoption has been uneven. Some states have incorporated elements into building codes. Most haven’t. The gap between what we know about indoor air and what we require of it remains vast.
Recent reporting reinforces the urgency. A March 2025 article in The Guardian highlighted research from University College London showing that poor indoor air quality in UK schools was associated with measurable declines in student test performance — not from dramatic pollution events, but from the routine, chronic inadequacy of ventilation in aging buildings. The findings echoed earlier work from Harvard and Lawrence Berkeley National Laboratory. The problem is global, persistent, and largely invisible because CO₂ is colorless and odorless. You can’t smell 1,500 ppm. You can only feel its effects — the headache, the sluggishness, the vague sense that something is off — and most people never connect the symptom to the cause.
So where does this leave us? The Grieve-Smith post is modest in scope — one person, one apartment, a handful of sensors over several years. But it illustrates something that large-scale studies often obscure: the intimate, hour-by-hour reality of the air we breathe at home. The data is unglamorous. Sleeping, cooking, hosting friends, opening and closing windows. And yet within that mundane record lies evidence of a systemic public health issue that affects virtually every person who lives in a building, which is to say virtually every person.
The technology to measure and manage indoor CO₂ exists. It’s affordable. It works. What doesn’t exist, at least not yet, is the political will to treat indoor air quality with the same seriousness we apply to outdoor air quality, drinking water, or food safety. The Clean Air Act regulates what comes out of smokestacks and tailpipes. Nothing equivalent governs the air inside the places where Americans actually live and breathe.
That absence is not an oversight. It’s a choice. And the data — whether from a Harvard lab or a blogger’s bedroom — keeps making the case that it’s the wrong one.
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