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I recently found this article by EWG, putting limonene-containing cleaners on the bad list of toxic household cleaners:

http://www.ewg.org/cleaners/hallofshame/

The claim is that limonene reacts with trace levels of indoor ozone to produce formaldehyde. Yet, citrus oils can be around 90% limonene or d-limonene. Does this mean that everything citrus or pine is dangerous now? Including orange essential oils, orange juice, orange scented diffusers and room sprays, candles with citrus oils, etc?

  • To be precise the limonene cleaner isn't on their toxic household list, but on their 'Greenwashing' list. They say "That the oils are derived from citrus implies safety, but sprayed into the air, they can react with trace levels of ozone air pollution to form ultra-fine particles that penetrate deep into the lungs and formaldehyde, which the U.S. government classifies as a known human carcinogen. The California Air Resources Board advises people to limit the use of citrus- or pine oil-based cleaners on smoggy days to avoid exposure to particulates and formaldehyde." – THelper Sep 20 '14 at 11:07
  • So limonene isn't "green"? – Sabotenderizer Sep 20 '14 at 19:02
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Yes, limonene is a terpene and terpenes react with ozone to form formaldehyde and other micro-particle pollutants. Limonene itself isn’t toxic and hasn’t been found to be harmful, but some people are sensitive to it’s various oxidation products causing skin irriation or respiratory problems.

Research has shown that:

when people use the [terpene-based cleaning] products under ordinary circumstances, their exposure to ethylene-based glycol ethers, formaldehyde and fine particles will normally not reach guideline values (source)

However the same study also says that under certain circumstances it is possible that levels of formaldehyde or microparticles exceed legal limits. Examples of such circumstances are:

  • using ozone-generating devices in combination with terpene-based air fresheners
  • being a professional cleaner with prolonged contact to (reactants of) terpene products
  • being exposed to other sources emitting formaldehyde (e.g. building materials, some furniture, glues, burning wood or gas).
  • bad ventilation

Apparently many daycare-centers in California were found to have levels of formaldehyde above the state's legal limit, but I’m not sure how strict Californian guidelines are. AFAIK there haven’t been studies on how harmful this is.

As to why exactly EWG placed the mentioned product on their Hall of Shame 'Greenwashing' list, my guess is that it is because of the rather high level of d-limonene.

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