Every flame is a particle source. Even the cleanest-burning candle deposits soot at some rate; lower-quality candles (paraffin wax with synthetic fragrance), poorly trimmed wicks, and any kind of incense are dramatically worse. A single stick of cone incense in a small room produces PM2.5 levels routinely above 200 µg/m³ for the duration of the burn.
Derudi et al. measured combustion products from common European candle styles and found scented paraffin candles emit roughly 3× the PM mass of unscented natural-wax candles. Lin's incense work documents complex VOC and PAH emissions, including known carcinogens.
Indoor air does not need to be combustion-free to be healthy. The threshold concern is duration and density. Burning one beeswax candle at dinner is unlikely to produce sustained elevation; burning six paraffin candles in a closed bedroom every night for a year is a meaningfully different exposure profile.
Practical guidance: ventilate during and immediately after burning, choose natural-wax candles with cotton wicks if possible, never burn incense in a bedroom, trim wicks to 1 cm before each light. The dashboard's "candle-burst" detection identifies the characteristic PM2.5-with-trace-VOC fingerprint and suppresses the false-alarm "cooking detected" hint.
References
- EPA - Indoor air quality basics www.epa.gov
- Lin et al. - Incense burning emissions pubmed.ncbi.nlm.nih.gov
- Derudi et al. - Indoor candle combustion products doi.org
- EPA - Improving indoor air quality www.epa.gov
