Source · product
Unvented heaters
Also: unvented heater, flueless heater, portable kerosene heater, unflued gas heater, paraffin heater
Heating appliances burning gas, kerosene or solid fuel that discharge their combustion products into the room instead of through a flue. Everything the flame makes stays indoors, and the appliance runs for hours at a time in cold weather with the windows shut.
Where it comes from indoors
An unvented heater is the worst combination of the variables that decide indoor combustion exposure, and it is worth naming why. The combustion products enter the room by design rather than by failure. The appliance runs continuously, not for the minutes a meal takes. It runs in the season when ventilation is lowest, because nobody opens a window in the cold. And it is used in the room people occupy, often the bedroom.
The emissions are those of the fuel and the flame: nitrogen dioxide from any flame, carbon monoxide when combustion is incomplete or the oxygen in a sealed room runs down, sulfur dioxide from kerosene containing sulfur, and particles from the wick.
Water is part of the emission too — combustion produces it — so an unvented heater raises humidity in the room it warms, which in a cold dwelling puts moisture on the coldest surface.
What the studies report
| Linked to | Effect | Population | Study design | Source | Checked |
|---|---|---|---|---|---|
| Nitrogen dioxide (NO₂) | ratio 1.8 (1.6–2.1) | school children · classroom | double-blind, cluster-randomised crossover trial in Australian school classrooms: low-NOx unflued gas heaters against non-indoor-emitting flued gas heaters, NO2 and formaldehyde measured with passive diffusion badges, children recorded symptoms, medication use and lung function twice daily | F-0689 Respiratory Health Effects of Exposure to Low-NOx Unflued Ga… (2010) | machine |
| Formaldehyde (CH₂O) | difference 9.4 (5.7–13.1) | school children · classroom | double-blind, cluster-randomised crossover trial in Australian school classrooms: low-NOx unflued gas heaters against non-indoor-emitting flued gas heaters, NO2 and formaldehyde measured with passive diffusion badges, children recorded symptoms, medication use and lung function twice daily | F-0690 Respiratory Health Effects of Exposure to Low-NOx Unflued Ga… (2010) | machine |
| Nitrogen dioxide (NO₂) | OR 1.16 (1.01–1.34) | school children · classroom | double-blind, cluster-randomised crossover trial in Australian school classrooms: low-NOx unflued gas heaters against non-indoor-emitting flued gas heaters, NO2 and formaldehyde measured with passive diffusion badges, children recorded symptoms, medication use and lung function twice daily | F-0691 Respiratory Health Effects of Exposure to Low-NOx Unflued Ga… (2010) | machine |
| Nitrogen dioxide (NO₂) | OR 1.38 (1.04–1.83) | school children · classroom | double-blind, cluster-randomised crossover trial in Australian school classrooms: low-NOx unflued gas heaters against non-indoor-emitting flued gas heaters, NO2 and formaldehyde measured with passive diffusion badges, children recorded symptoms, medication use and lung function twice daily | F-0692 Respiratory Health Effects of Exposure to Low-NOx Unflued Ga… (2010) | machine |
| Nitrogen dioxide (NO₂) | median 32 | school children · classroom | double-blind, cluster-randomised crossover trial in Australian school classrooms: low-NOx unflued gas heaters against non-indoor-emitting flued gas heaters, NO2 and formaldehyde measured with passive diffusion badges, children recorded symptoms, medication use and lung function twice daily | F-0693 Respiratory Health Effects of Exposure to Low-NOx Unflued Ga… (2010) | machine |
| Particulate matter (PM₂.₅ and PM₁₀) | mean 29.97 | — · home | measurement study: 24-hour indoor and outdoor samples of PM10, PM2.5, particle strong acidity, sulfate, nitrate, ammonia, nitrous acid and sulfur dioxide at 281 homes in southwest Virginia and Connecticut in winter and summer, 74 winter homes using kerosene heaters | F-0170 Indoor, outdoor, and regional summer and winter concentratio… (1999) | machine |
| Carbon dioxide (CO₂) | the sentence opens with a reference to Figure 4, but states its own result; the heaters are kerosene heaters per the housing table, filed under unvented-heaters | households · living room | field investigation of CO2 concentration in 24 occupied houses in urban Japan (October 2020 to the following winter) with a questionnaire survey, comparing houses with and without combustion (kerosene) heating | F-0525 Hattori et al. Investigation of Indoor Air Quality in Residential Buildings… (2022) | machine |
Each row is one Finding — a sentence as printed in one study, recorded with its page in the Coridum Knowledge Base. Follow the Finding number for the exact wording and the citation. Checked says who verified the row: machine means every automatic check passed and the surrounding text was read; person means someone read the sentence in the document. Rows are re-checked by a person over time. The studies are curated in the Coridum Knowledge Base; the reading of them together is a Claim, shown separately.
Cite this page
Coridum Research. Unvented heaters. Coridum Knowledge G-0136, 16 September 2026. https://coridum.com/k/G-0136/