https://www.livescience.com/amp/52080-essential-oils-science-health-effects.html
Inhalation: absorption and detoxification. Similar enzymes occur in the alveolar cells, modifying any chemicals absorbed through inhalation. There is almost a direct entry into the lung cells for lipophilic molecules in the essential oils as there is only one cell membrane thickness to traverse. This is why the effect of diffusers or simply breathing in fragrances added to bathwater can be substantial. Damage can occur to the lungs due to excessive use of certain chemicals in essential oils, but the actual concentration has not been worked out and very few studies are available (Cooper et al., 1995). The risk of respiratory cancer in workers after 5 years of exposure to industrial terpenes from conifers is greatly increased (Kauppinen et al., 1986). However, in another study, exposure to -pinene enantiomers for 20 minutes at 10–450 mg/m3 did not cause acute changes in lung function (Falk et al., 1990). Studies on the absorption of inhaled essential oil components are very rare, but one showed that 1,8-cineole was rapidly absorbed from eucalyptus essential oil, with plasma concentrations at their peak after 18 minutes (Jaeger et al.,1996). The direct entry of lipophilic components from essential oils via the olfactory mucosa is quite substantial and they can act like anesthetics very rapidly. Entry via the blood-brain barrier can also be substantial, especially in neonates and young children where it is undeveloped.