1,8-Cineole (Eucalyptol)

Structure and physical constants

1,8-Cineole is a monoterpenoid: C10H18O at about 154.25 g/mol, a bicyclic ether in which an oxygen bridges the 1 and 8 positions of a p-menthane skeleton to give a rigid oxabicyclic cage. The ether oxygen is the whole story — it raises the molar mass and the boiling point relative to the C10H16 monoterpenes, gives measurable water solubility (which is why cineole is prominent in hydrosols and not only in the oil), and makes the molecule a clean substrate for oxidative metabolism. Atmospheric boiling point is about 176 to 177 degrees Celsius. The name eucalyptol is the same compound; 1,8-cineole is the preferred name because 1,4-cineole also exists and is a different substance.

Sources: Compiled from public compound databases (PubChem 2026*

Respiratory effects — the human trials contested human data

Cineole carries better human evidence than any other compound on this shelf. Juergens and colleagues ran a double-blind placebo-controlled trial of 1,8-cineole in bronchial asthma and reported anti-inflammatory activity, with steroid-sparing effect as the clinical endpoint. Worth, Schacher and Dethlefsen ran a randomised placebo-controlled trial of concomitant cineole therapy in chronic obstructive pulmonary disease and reported a reduction in exacerbations. Mechanistically cineole is described as mucolytic and secretolytic, and as inhibiting inflammatory cytokine production in respiratory tissue. This is the evidence base underneath the very old practice of inhaling eucalyptus and eucalyptus-type volatiles for respiratory complaints — including the traditional smoke inhalation recorded for Imphepho, which is a cineole-rich material. The trials are on isolated 1,8-cineole administered in defined doses, not on inhaled plant smoke, and that distinction matters: smoke condensates from burning plant material have been shown to contain different compounds than solvent extracts of the same plant, so the trial evidence does not transfer directly to a combustion route.

Contested — caveat. The human trials used defined doses of isolated 1,8-cineole. They do not establish equivalent effect from inhaling smoke or vapour of a cineole-containing plant, where the delivered dose is unknown and combustion generates compounds absent from the intact material.

Sources: Juergens UR 2003 · Worth H 2009* · Lourens ACU 2008 · Van Kush Family Research Institute (operator) 2026*

Cytochrome P450: cineole is a CYP3A substrate contested in vitro

Miyazawa, Shindo and Shimada showed that 1,8-cineole is oxidised by CYP3A enzymes in rat and human liver microsomes, identifying CYP3A4 as a principal catalyst of its hydroxylation in human liver. The direction of that relationship is worth stating carefully, because consumer-facing writing usually gets it backwards: cineole is documented as a SUBSTRATE of CYP3A, which is a statement about how the body clears cineole. Whether cineole meaningfully inhibits or induces CYP3A at human exposure levels — and therefore whether it could alter the clearance of a co-administered drug — is a separate question that microsomal substrate work does not answer. 1,8-cineole does appear in the induction literature for some enzyme systems in animal models. Anyone reasoning about interactions should go to the CYP450 shelf and should note that the compound with the most-cited CYP3A INHIBITION data in this botanical space is piperine from black pepper, not cineole.

Contested — caveat. Substrate relationship, not an inhibition constant. Microsomal metabolism data do not establish a clinically relevant drug interaction. Do not read this as an interaction warning or as its absence.

Sources: Miyazawa M 2001*

Toxicity and handling, stated plainly contested

Cineole is not a benign compound at arbitrary dose, and this belongs on an industry page rather than being softened. Eucalyptus oil and 1,8-cineole ingestion is a recognised paediatric poisoning, with central nervous system depression and seizure reported after ingestion of small volumes of concentrated oil by small children; concentrated cineole is also a mucous-membrane and skin irritant. The relevant industrial consequences are containment, child-resistant packaging and dilution discipline for anyone handling cineole-rich oils, and awareness that a hydrosol carries cineole too, at much lower concentration. This is a handling and packaging fact about a compound already in hand, not medical advice.

Contested — caveat. Toxicology summary compiled from general reference sources rather than from a specific primary case series cited here; treat the concentration thresholds as directional and consult a current toxicology reference for numbers.

Sources: Compiled from public compound databases (PubChem 2026* · Juergens UR 2003

Where 1,8-cineole actually is, and at what percent contested

The operator's Helichrysum odoratissimum figure makes cineole one of the two defining compounds of that material alongside alpha-pinene, and the review literature names 1,8-cineole and alpha-pinene together as the constituents common to the South African Helichrysum species surveyed.

Contested — caveat. Chemotype figures. Rosemary and eucalyptus both have cineole-poor chemotypes sold under the same common name; a composition spec, not a species name, is what tells you what is in the drum.

Sources: Van Kush Family Research Institute (operator) 2026* · Lourens ACU 2008 · Compiled from public compound databases (PubChem 2026* · Booth JK 2019

Industry notes

For a formulator, cineole is the monoterpenoid that survives into the hydrosol — the aqueous distillation byproduct the operator's document flags as valuable for toners and sprays is cineole-bearing, and that is a real, saleable second product from the same distillation run rather than a waste stream. For a lab tech, cineole is a well-behaved GC analyte and its ratio to alpha-pinene is a useful chemotype fingerprint for Helichrysum and rosemary material. For a buyer, insist on a composition certificate rather than a species name.

Sources: Van Kush Family Research Institute (operator) 2026* · Lourens ACU 2008