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Green, natural and unscented / Measured in people or real products
The unscented 'plain' paraffin candle used as this study's own combustion reference produced the third-highest formaldehyde level of the six candles tested when lit (925 ppb), higher than two of the five scented candles.
Moderate evidence.
Caveats
Single lab, six products, no replication; wax type (paraffin vs. palm) did not track cleanly with formaldehyde output either, so this alone doesn't identify which factor (wax, wick, dye, or fragrance) actually controls it.
Source: Ahn et al. 2015, Journal of Hazardous Materials Link to this fact
Information, not medical advice. See also: myths we won’t tell you.