Don’t take our word for it. Here’s every fact on this site, with its evidence grade, its caveats, and a link to the source, so you can check us.
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.
Indoor air / Review of other studies
Scent chemicals released by perfumes and scented products can react with ozone in indoor air to form new pollutants, including formaldehyde and fine particles.
Strong evidence.
Caveats
Formation is well established (Fiedler 2005: 40 ppb O3 + a limonene/alpha-pinene-containing VOC mix -> HCHO 13 -> 40 µg/m³, UFP 2,500 -> 46,000 cm-3; Singer 2006; Rossignol 2013; Destaillats 2006 gives the underlying per-ozone yield). Amounts depend on dose and ozone; the harm at real indoor levels is not shown. A far more rigorous, independent toxicologist review (Wolkoff & Nielsen 2017) reaches the same qualitative chemistry conclusion via its own literature synthesis and adds a quantitative caution: in the two largest human tests to date (Fiedler 2005, n=130; Fadeyi 2015, n=71), the resulting reaction mixture caused no significant airway/eye effects, and its own threshold estimate is that >200 µg/m³ ozone at high limonene would be needed before sensory effects are expected. Cite primaries for numbers.
Source: Alblooshi 2025, Frontiers in Toxicology Link to this fact
Indoor air / Measured in people or real products
In week-long air measurements across 11 European cities, the scent chemicals limonene and alpha-pinene were the compounds most concentrated in homes compared with workplaces (about 4x), attributed to perfumes, cleaning and fragranced products.
Strong evidence: Source attribution by the authors, not measured per product.
Source: Geiss 2011, Atmospheric Environment Link to this fact
Indoor air / Other evidence
Terpene ingredients in scented cleaners, air fresheners, perfumes, colognes and essential oils (limonene, pinene, linalool, linalyl acetate) are listed as ozone-reactive indoor sources that make formaldehyde and tiny particles when ozone is present.
Strong evidence.
Caveats
Table 1 is a literature summary; perfumes and essential oils rest on three chamber citations (Chao 2005, Karamalegos 2005, Shu 1997). No product was measured in this paper and it gives no product-specific numbers.
Source: Weschler 2006, Environmental Health Perspectives Link to this fact
Information, not medical advice. See also: myths we won’t tell you.