Every fact on this site, with its evidence grade, its caveats, and a link to the source.
We start you on the strong ones: findings that have been replicated or come from one large, well-controlled study. Moderate and weak facts are here too, labeled for what they are. Use “Link to this fact” to share a single card.
What you can do / Review of other studies
For fragrance-allergic people, dermatologists recommend removing diffusers, scented candles, room deodorizers, incense, and wax melts, switching household cleaners, wipes, and laundry products to fragrance-free, and bringing your own shampoo to the salon and your own oil to massages.
Moderate evidence: Expert advice; the salon point rests on a single exposure keeping skin flared up to ~4 weeks (clinical observation).
What you can do / Measured in people or real products
Choosing an unscented candle over a scented one made from the same wax measurably lowers the release of combustion pollutants (VOCs, PAHs, formaldehyde), based on this chamber study, though unscented candles still produce carbon dioxide, carbon monoxide, nitrogen oxides, and ultrafine particles from ordinary combustion.
Moderate evidence: Demonstrated for purpose-built research candles across 4 wax types and 5 fragrance families, not verified for any specific retail candle brand.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
What you can do / Measured in people or real products
Ultrafine particle release from a burning candle is highest in the first 30-60 minutes after lighting, on average 3 to 5 times higher than during steady burning, so ventilating a room in the first hour after lighting candles removes more particles than ventilating later in a burn.
Moderate evidence.
Caveats
Comparison is described qualitatively in the text ("on average a factor of 3-5 higher"), not given as a precise table value; based on the ignition-phase behavior of the chamber's continuous particle monitor, not a home-scale test.
Source: Salthammer 2021, Environment International Industry-funded Link to this fact
What you can do / Animal study
If burning scented candles indoors, ventilate the room (open window or extraction) rather than burning in a sealed, air-conditioned space, and prefer shorter burn times: in this study the worst inflammatory and histopathological outcomes came from the longest daily exposure (6 h/day) in a fully sealed room.
Moderate evidence: Inferred from a rat model with an unmeasured indoor dose; the specific ventilation rate needed to meaningfully reduce exposure was not tested.
Source: Mohamed et al. 2025, Frontiers in Public Health Link to this fact
Information, not medical advice. See also: myths we won’t tell you.