At a glance
| Chemical family | The smallest common cyclic volatile methylsiloxane (cVMS); a volatile silicone. Filed here as a VOC for its airborne behaviour, alongside its sibling D5 |
| CAS number | 556-67-2 |
| Classification | Not a carcinogen (not IARC-classified). EU Substance of Very High Concern — persistent & bioaccumulative (PBT / vPvB); harmonised classification "suspected of damaging fertility"; EU endocrine-disruptor listing (contested) |
| Where you encounter it | Personal-care products (antiperspirants, hair and skin care) at lower levels than D5; an industrial silicone feedstock |
| Sleep micro-environment relevance | Evaporates off skin and hair into bedroom air like D5, but at lower product concentrations; being phased out of cosmetics |
| Activated carbon capture | Not the lever — product reformulation (already underway) and lighter use of silicone products address it |
What it is
D4 is octamethylcyclotetrasiloxane, the smallest of the common cyclic volatile methylsiloxanes — the silicone family that also includes D5 and D6. It is a volatile, oily fluid used as a carrier and emollient in personal-care products and, more heavily, as a building block for manufacturing silicone polymers. In finished cosmetics it now appears at much lower concentrations than D5, which is the dominant cyclic siloxane in products. Peer-reviewed — Dudzina et al. 2013
The single most important correction this page makes is about its hazard label. D4 is not a carcinogen — it has not been classified as one by IARC or other major bodies, and calling it "Group 2B," as some lists do, is an error. Inferred — D4 is absent from IARC carcinogen classifications; the common "Group 2B" label is a misattribution and is corrected here Its actual regulatory concerns lie elsewhere.
How it relates to the bedroom
The same airborne route as D5
For the sleep environment, D4 behaves like its sibling: it is volatile, so it lifts off skin and hair after a product is applied and enters the air you breathe, rather than depositing on sheets. The practical difference is concentration — because D5 is the predominant cyclic siloxane in modern products and D4 the minor one, the airborne D4 in a typical bedroom is lower than the D5. Peer-reviewed — Dudzina et al. 2013 The pathway, though, is identical, which is why D4 belongs beside D5 in the volatile-organic family rather than among the personal-care residues that linger on fabric.
Why D4 is the more-regulated sibling
Here the two diverge. D5's toxicology is relatively reassuring; D4's is not, and regulators moved against it first. The EU designated D4 a Substance of Very High Concern on the grounds that it is persistent, bioaccumulative and toxic in the environment (PBT) and very persistent and very bioaccumulative (vPvB) — and it carries a harmonised classification as suspected of damaging fertility, plus a listing as an endocrine disruptor. Regulatory — EU REACH Annex XVII entry 70 / ECHA SVHC That reproductive flag has experimental support: unlike D5, D4 shows weak estrogenic activity, binding the estrogen receptor and producing increased uterine weight and estrous-cycle changes in animal studies. Peer-reviewed — Marrugo-Padilla et al. 2024 In fairness, the endocrine-disruptor classification is contested, with a 2024 mechanistic review arguing that D4 lacks estrogen-pathway disruptive potential — so the science is not fully settled. Speculation — D4's endocrine-disruptor status is scientifically debated
Restricted, like D5, for the environment first
D4 sits in the same cosmetic restriction as D5: under EU REACH it may not be placed on the market in wash-off cosmetics at 0.1% or more, a limit later extended to leave-on products, with environmental persistence the primary driver. Regulatory — EU Commission Regulation 2018/35, Annex XVII entry 70 The combination of PBT/vPvB plus the fertility flag is why D4 is being designed out of personal-care formulations faster than its larger sibling. Inferred — the stacked PBT + reproductive concerns drive faster phase-out of D4
What the research says
- Not a carcinogen. Not IARC-classified; the "Group 2B" label is a misattribution. Inferred — corrected here
- EU Substance of Very High Concern. Persistent and bioaccumulative (PBT / vPvB); environmental driver of its restriction. Regulatory — ECHA / EU REACH
- Suspected fertility toxicant. Weak estrogenic activity and reproductive effects in animals — unlike D5. Peer-reviewed — Marrugo-Padilla et al. 2024
- The minor cyclic siloxane in products. Present at much lower levels than the dominant D5. Peer-reviewed — Dudzina et al. 2013
What helps reduce it
Use fewer silicone-heavy hair and skin products. The same lever as for D5 lowers airborne cyclic siloxanes generally. Inferred
Let reformulation do the work. EU restrictions are removing D4 from cosmetics; choosing compliant products accelerates the shift. Regulatory — EU Commission Regulation 2018/35
Ventilate after getting ready. As with D5, airflow disperses the brief post-application spike. Inferred
What does NOT help
- Treating D4 as a cancer risk. It is not a carcinogen; the real flags are environmental persistence and fertility. Inferred
- Laundering bedding. D4 is airborne, not a fabric residue, so washing sheets does nothing for it. Inferred
Open research questions
- Whether D4's weak estrogenic and reproductive effects in animals translate to any human risk at airborne personal-care exposure levels. Speculation
- How quickly product reformulation is reducing residual D4 in cosmetics and indoor air. Speculation
Citations
- Commission Regulation (EU) 2018/35 amending Annex XVII to REACH (entry 70); ECHA SVHC listing. D4 (CAS 556-67-2) + D5 restricted to <0.1% in wash-off cosmetics from 31 Jan 2020; D4 = SVHC (PBT/vPvB); harmonised CLP "suspected of damaging fertility"; very toxic to aquatic life. legislation.gov.uk / ECHA Regulatory
- Marrugo-Padilla A, et al. (2024). Toxicokinetic profiles and potential endocrine disruption effects at the reproductive level promoted by siloxanes. J. Appl. Toxicol.. D4 binds ER-α with low estrogenic activity; increased uterine weight and estrous-cycle alterations in rats; reproductive concern (contrast: D5 does not bind the estrogen receptor). Via Consensus. Reference record Peer-reviewed
- Dudzina T, et al. (2013). Concentrations of cyclic volatile methylsiloxanes in European cosmetics and personal care products. Environment International. D5 the predominant cVMS; D4 present at much lower levels — the minor cyclic siloxane in modern products. Via Consensus. Reference record Peer-reviewed
Frequently asked questions
What is D4?
D4 — octamethylcyclotetrasiloxane — is the smallest of the common cyclic volatile silicones (cVMS), a close relative of D5. Like D5 it is volatile and was used as a carrier and emollient in personal-care products such as antiperspirants, hair and skin products, and as a building block for making silicones. In modern cosmetics it is present at much lower levels than D5, which dominates, and it is being phased out.
Is D4 a carcinogen?
No. D4 has not been classified as a carcinogen by IARC or other major bodies, and it should not be labelled one — a common error. Its regulatory concerns are different: the EU classifies it as a Substance of Very High Concern for being persistent and bioaccumulative in the environment (PBT and vPvB), and it carries a harmonised classification as suspected of damaging fertility. The EU has also listed it as an endocrine disruptor, though that classification is scientifically contested.
How does it differ from D5?
They are close siblings and behave similarly in the bedroom — both are volatile silicones that evaporate off skin and hair into the air. The differences are in degree and concern: D4 is present at lower levels in products (D5 is the dominant one), but D4 draws more regulatory concern because, unlike D5, it shows reproductive and weak estrogenic effects in animal studies and is a suspected fertility toxicant. So D4 is the smaller-but-more-restricted of the two.
Should I be worried about it?
For everyday airborne exposure, the human-health concern is modest and centers on reproduction rather than cancer, and D4 is already being removed from cosmetics under EU rules. The strongest reasons to reduce it are environmental persistence and the precaution around fertility. The practical steps are the same as for D5: fewer silicone-heavy hair and skin products and good ventilation; product reformulation is steadily lowering it regardless.
Related compounds
Embr is a sleep environment company researching and addressing the chemistry of the bedroom. Research and product development in progress.
Last reviewed 2026-06-27. If you find a factual error, contact us.