Bioavailability refers to the proportion of an administered dose of a substance which reaches the systemic circulation, and depends on the substance, the method, dose and frequency of administration. Bioavailability will be affected by the individual recipient, including their general health and wellbeing, age, ethnicity, metabolism and factors such as the environment, hirsutism and skin integrity. In general aromatherapy practice, gender is also a factor.
When essential oils are administered via the skin, absorption rates differ between individuals, between different areas of the body and between chemical constituents. Whilst the skin allows for slower absorption than the gastrointestinal tract, the fact that the skin is the body’s largest organ means that there will, overall, be considerable absorption and systemic circulation. The carrier medium used for massage – usually a vegetable oil – results in slower absorption than when essential oils are added to water,
particularly when the water is hot, which causes vasodilatation and opens up the hair shafts. However, massage stimulates the local circulation, which aids absorption, although essential oils which are more volatile will evaporate into the air quickly, reducing the amount available for dermal absorption.
Applying essential oils to warm skin, either in water or as a massage, will also enhance absorption. Neat oil absorbs more quickly than essential oil diluted in carrier oil, due to percutaneous absorption, and results in higher concentrations of the chemical constituents in the bloodstream, increasing the risk of toxicity (see Chapter 4). The essential oils mix with skin lipids, reducing the barrier function of the skin by making them more hydrophilic, and are therefore facilitated to cross into the dermal layer. Trauma to the skin and dermatitis conditions such as eczema or psoriasis reduce the skin integrity and allow faster absorption of essential oils, and the resulting immune response may cause inflammation. Even stress can affect the ability of the skin to absorb essential oils appropriately, either expediting or inhibiting absorption. Although most essential oil constituents will be fully absorbed into the circulation within 24 hours, some will be retained for up to 72 hours in the epidermis before crossing to the dermis and then into the systemic circulation. There is some suggestion that human skin is more permeable in the evenings and at night rather than in the morning (Yosipovitch et al. 1998), and pigmented skin appears to have a better barrier function, implying slower absorption than in pale-skinned women. Covering the skin after administration may also encourage greater absorption.
Neonatal skin is much thinner than adult skin and dermal application should be completely avoided until at least three months of age. It is not the purpose of this book to consider aromatherapy in babies, but midwives and doulas should be mindful of these issues and take steps to advise women accordingly. Tisserand and Young (2014) suggest that children aged 3–24 months should receive no more than 0.25%; for those aged 2–6 years a maximum of 1% is recommended; 1.5% is advised for children up to the age of 15 years. Any commercial products intended for infants should only contain essential oils if they have been dispersed in a water-soluble medium.
Essential oils administered via inhalation pass to the respiratory tract and reach the pulmonary alveoli, which facilitate the passage of the small molecules into the bloodstream and subsequently pass into the central nervous system (CNS) relatively quickly. This has implications for women with CNS conditions such as epilepsy (see Chapter 4). There is only a single membrane for the lipophilic molecules to traverse into the lungs, so inhalation of essential oils can produce a substantial – and possibly an adverse – reaction. Even inhaling vapours from adding essential oils to the bath water can have a crucial effect. Entry via the blood–brain barrier can also be significant. The rate and depth of breathing influences absorption, so
women in the transition stage of labour, who may be hyperventilating, will absorb more essential oil than when they are breathing normally. Prolonged exposure to vapours (more than 30 minutes) may not only trigger the minor side-effects associated with the aromas but may also have more widespread systemic effects. When essential oils are administered via inhalation, whether as the primary method or as a secondary effect of dermal administration, it is difficult to assess the dosage. This is particularly so when certain constituents are more volatile than others and are more rapidly lost to the atmosphere.
Essential oils are lipophilic and, once absorbed into the body, generally work in a similar way to other fat-soluble substances. Distribution of essential oils in the general circulation depends on the degree of their lipophilic activity.
Essential oil constituents penetrate the blood–brain barrier and can have a direct effect on various receptors in the brain. They also reach the liver and kidneys rapidly, whereas their impact on skeletal muscle is less significant.
Essential oils will be stored in adipose tissue for some considerable time, depending on the oil and its various constituents, during which time they have little or no pharmacological action. Retention of the oil chemicals in the fat will obviously be considerable in obese women and significantly less so in those who have a very low body mass index.
Essential oil molecules which have metabolised will eventually bind to plasma-binding proteins in the same way as drugs. This inactivates any further pharmacological action, but the molecules can also not be transformed and excreted, essentially prolonging their retention by the body.
Interaction with drugs competing for the same protein-binding sites may also occur. However, this is not a major issue when essential oils are administered in clinically appropriate doses via the skin or inhalation, and refers more significantly to oral or even intravenous administration. However, some essential oil constituents raise the production of metabolic enzymes which may increase or decrease the rate of metabolism and exacerbate the potential for drug–oil interaction. This is particularly the case with the cytochrome P450 enzyme found in the liver and small intestine, although essential oils would have to be administered in large doses, primarily via inhalation or by mouth, to have any real adverse effects (this is unlikely to occur with dermal administration). There is also a theoretical possibility of some essential oil constituents inhibiting or exacerbating other chemicals when present in large amounts. These include monoamine oxidase inhibitors, selective serotonin reuptake inhibitors, anticoagulants (e.g. warfarin and heparin) and tyramine, but the limited amount of evidence refers largely to oral administration and is not explored here. Despite this, there are anecdotal reports of pathological complications occurring when essential oils have been administered in people taking various drugs and it is wise to be extremely cautious when using aromatherapy for pregnant women with medical conditions for which they require medication (see Chapter 4).
Different rates of metabolism are seen with different chemical constituents which are eventually excreted via the kidneys, liver, lungs, skin and intestines. All essential oil constituents in the bloodstream pass through the kidneys and are excreted in the urine but, for those which adhere strongly to the plasma proteins, filtration and excretion will be slower. Essential oils which are inhaled will be excreted only in very small amounts by the kidneys, pulmonary excretion being the main method. Small amounts of some constituents may pass to the sweat, faeces, saliva and breastmilk, and in this latter case, can be ingested by the baby.