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Description of key information

Short description of key information on absorption rate: 
Dermal absorption from in vitro studies: 21% maximum absorption (worse case) Real life absorption typically ~1-2%. Evaporation half life around 12 seconds from skin.

Key value for chemical safety assessment

Bioaccumulation potential:
no bioaccumulation potential
Absorption rate - oral (%):
90
Absorption rate - dermal (%):
21
Absorption rate - inhalation (%):
75

Additional information

ABSORPTION

Ethanol has a low molecular weight (46.07) and is highly soluble in both water and lipid, allowing absorption across the surface of the gastrointestinal (GI) tract, the lungs and skin. Following ingestion, absorption of ethanol begins immediately with greater than 90% of the consumed dose being absorbed by the GI tract.  The consumption of two alcoholic beverages (approximately 20g ethanol) results in a maximum BEC of approximately 300 mg ethanol/L within one hour; the concentration of ethanol in blood then rapidly declines, reaching endogenous levels after several hours.

Ethanol can also be absorbed by inhalation.  A study investigated the respiratory uptake of methanol (a similar alcohol to ethanol) in male volunteers using a single exposure concentration of 100ppm and established that over a 10 minute period of exposure, uptake is around 61% of the inhaled concentration (Kumagai, 1999).  The authors concluded that the results of this study (which examined a number of solvents) when correlated with partition co-efficients supports the hypothesis that solvent absorbed in the mucous layer of the respiratory tract is removed by the bronchial blood circulation  An approximate figure of 60% absorption of inhaled ethanol is supported by their human studies with relatively high levels of exposure, ranging from 5000 to 10,000 ppm ethanol (Lester, 1951).  A recent study using lower levels of ethanol exposure (25 – 1000 ppm) reported a value of between 70 to 80 % absorption (Tardif, 2004), which may be more representative of occupational exposure levels. Once absorbed by this route, the degree of ethanol retention is generally low due to the ‘wash-in-wash out effect’ observed with water soluble chemicals.  Seeber et al., (1994) exposed 24 volunteers  to ethanol at 80, 400 and 800 ppm for 4 hours with resulting blood ethanol levels of 0.25, 0.85 and 2.1 mg/l respectively.  A PBPK model for inhaled ethanol was established by Lester and Greenburg (1951) for mice and rats which has subsequently been adjusted and applied to humans by Gentry et al., (2010).  Using the adapted PBPK model, it has been calculated that at exposure levels of up to 9500 mg/m3 (5000 ppm) for 8 hours (ventilation rate of 9 l/min) ethanol metabolism was not saturated and the liver was able to metabolise the ethanol at the rate that it entered the body. Using this data, it is possible to estimate that at occupational ethanol levels of 1000 ppm for a worker inhaling 10 m3 per day with an absorption rate of 60%, pulmonary absorption would be equivalent to 11.4g of ethanol.   However, for risk assessment purposes, it is more relevant to look at peak blood ethanol concentrations and the ‘area under the curve’ (concentration times duration of exposure-AUC).  For inhalation, the PBPK model estimated that the AUC for an 8hr 1000ppm exposure is 45mg.hr/l.  It is also possible to make a similar calculation from the Seeber data, from which it is possible to derive a function of BEC(blood ethanol concentration)=exposure (ppm) × 0.0029 (with a 7% error for 95% confidence).  This would give a BEC of 2.9mg/l and an AUC over 8hrs of 23mg.hr/l (lower but not dissimilar).  This can be compared to a similar estimation from the oral data.  Assuming  instantaneous distribution and linear kinetics for elimination, a dose of 11.4g would give a BEC of 302mg/l (assuming a volume of distribution of 0.54l/kg and a 70kg person).  If this is eliminated at a rate of 127mg/kg/hr (8.9g/hr), total elimination would occur in 11.4/8.9=1.29hrs.  The area under the curve is then 194mg.hr/l.  Calculating the at the other extreme (lighter person – 60kg, slower elimination – 83mg/kg/hr or 4.99g/hr) would lead to a peak BEC of 352mg/l, an elimination time of 2.28hrs and an AUC of 401mg.hr/l.  This demonstrates that a simple calculation of quantitative uptake based on inhalation concentration, duration and percentage uptake significantly overestimates the exposure by a factor of approximately 5-15x compared to measured data.

Data from animal studies has shown that following dermal contact, ethanol can penetrate the skin and be absorbed.  In a study to assess the skin penetration potential of ethanol, an in vitro study was carried out using excised pig skin and radiolabelled ethanol. Ethanol penetration was greater under occlusion conditions than non-occlusive conditions, as might be expected. Absorption rates were around 21% and 1% of applied doses respectively. (Pendlington, 2001).  The same study assessed the important of evaporation from excised pig skin. The evaporation half life was found to be 11.7seconds, suggesting that systemic doses of ethanol resulting from skin absorption following single exposures to ethanol will, under practical conditions, be very low due to rapid evaporation.  A third part of the same study assessed the potential for skin uptake of ethanol from the use of personal care products, human volunteers applied ethanol over their whole bodies delivered from an aerosol can. The mean dose of ethanol applied was just under 10g per subject. Ethanol could not be unequivocally detected in the blood of any of the 16 volunteers in the hour immediately after application. Lack of uptake has also been demonstrated in volunteer studies using ethanol based hand sanitisers.

In an in vitro study using guinea pig skin exposed to ethanol for 19 hours, the amount of ethanol penetrating the skin was only 1% of the total dose, and the dermal flux for ethanol in human skin (epidermis) in vitro was determined by the authors to be 0.57 mg/cm2/h (Gummer, 1986). Increases in the dose of ethanol did not alter the degree of skin penetration. However, occlusion was seen to significantly enhance penetration, increasing the dermal flux by a factor of 10.

A male human volunteer was exposed to ethanol vapour at 1900 mg/m3 in an exposure chamber for 3 hours. Exposures were carried out at different ventilation rates. Ethanol in blood samples remained consistently below the limit of detection of 2 mg/L. It was concluded that exposure to ethanol vapour at 1900 mg/m3 will not produce a significant blood alcohol concentration. Another study using five male exposed for six hours to air containing ethanol at concentrations of up to 1000 ppm produced a similar result. Ethanol was not detected in the blood of volunteers exposed to 250 and 500 ppm, whereas blood concentrations measured at 3 and 6 hr during exposure to 1000 ppm (about 1884 mg/m3) were 0.229 mg/100 ml and 0.443 mg/100 ml, respectively. Ethanol concentrations in expired alveolar air "reached a steady-state 3 h after the start of exposure" and ranged from 241 to 249 ppm in the volunteers exposed to air containing ethanol at 1000 ppm (suggesting that absorption of ethanol from the lungs is approximately 75%).

PBPK modelling work and experimental validation studies have shown that at concentrations up to 1000 ppm in humans, the liver can metabolise ethanol at the rate it enters the body at any realistic breathing rate. Blood ethanol concentrations remain very low and do not rise during exposure. Typically, exposure to 1000ppm ethanol vapour produces a blood ethanol concentration of 2.5 -3mg/L. The same holds true at 5,000 ppm when the breathing rate for a worker at rest is used. However, as breathing rate increases, the concentration of ethanol in the blood increases to a point that the capacity of the metabolic pathway is surpassed and ethanol blood levels consequently rise for the duration of the exposure. These results clearly indicate that ethanol levels in the blood stream will be very low at typically encountered exposures and also that accumulation only becomes significant at exposures associated with discomfort. Similar results have also been seen in rats where concentrations up to 10000ppm do not saturate metabolism, whilst 21000ppm clearly does.

DISTRIBUTION

Irrespective of the route of exposure, following absorption into the bloodstream, ethanol is distributed throughout the body with the final volume of distribution close to that of total body water, estimated as 50 – 60 % of lean body weight in adults. Ethanol perfuses organs with the greatest blood supply most quickly (brain, lungs and liver) and equilibrium between tissues and blood is generally achieved within 1 – 1.5 hr after ingestion (Bevan, 2009-J Tox Env Hlth B Crit Rev, 12(3), 188-205).

In a developmental neurotoxicity study, the levels of blood ethanol resulting from doses of 3, 4 and 5g/kg in 4 day old pups was found to be 205, 265 and 355mg/dL respectively (Lindquist, 2013).

In a toxicokinetic study designed to produce data to both validate and refine an existing PBPK model, Long-Evans pregnant and non-pregnant rats were exposed to vapour concentrations of ethanol of 5000, 10000 and 21000ppm. At the two lower concentrations, the blood ethanol concentrations rapidly reached a plateau of ~1.5 and 5mg/dL respectively. However, at 21000ppm, there was a continuous increase in concentration over the 6 hour exposure period, reaching a final value of ~220mg/dL. (Martin, 2014).

METABOLISM

Prior to absorption, ingested ethanol undergoes limited metabolism (first pass metabolism) in the stomach by gastric alcohol dehydrogenase.  The role of first-pass metabolism is, however, not relevant for exposure to ethanol via inhalation and dermal routes. Once absorbed, ethanol is metabolised, principally by the liver, which accounts for 92-95% of capacity with minor amounts metabolised in other tissues such as the kidney and lung (Crabb et al., 1987). A number of metabolic paths are available but only one is relevant to the low blood ethanol concentrations likely to result from either inhalation or dermal exposure and only this mechanism is described here.  In such cases, Ethanol metabolism in the liver is carried out in three steps, namely, (i) oxidation of ethanol to acetaldehyde (AcH), (ii) conversion of AcH to acetate and (iii) oxidation of acetate to carbon dioxide and water. In the first step, ethanol is converted to AcH by alcohol dehydrogenase (ADH) which occurs in the soluble fraction of liver cells (cytosol).  The conversion of ethanol to AcH by ADH is the rate-limiting step in ethanol metabolism as ADH has a low Michaelis-Menten constant (Km).  ADH has been shown to exhibit polymorphisms that affect functional activity (Pastino et al., 2000) accounting for ethnic variations in the pharmacokinetics of ethanol (Norberg et al., 2003).   In a study to examine the metabolism of ethanol and in particular the difference between humans with normal and humans with deficient ALDH, 60 volunteers were given a single oral dose of ethanol and the elimination kinetics determined by following subsequent blood ethanol levels with time. The data showed that there is a slight but not a significant difference in the ethanol elimination rates between those with normal ALDH and those with deficient ALDH.  In a study to examine the metabolism of ethanol and in particular the difference between humans with normal and humans with deficient ALDH and the contribution of first pass metabolism, male volunteers were given a oral doses of ethanol and the elimination kinetics determined by following subsequent blood ethanol levels with time. This was compared to similar doses given intravenously. The effect of fasting versus eating prior to dosing was also examined. The data showed that resultant blood ethanol concentrations were always lower by the oral route and that oral dosing on a full stomach greatly reduced oral uptake. The study also showed that deficient ALDH resulted in greatly increased blood acetaldehyde concentrations but not significantly increased ethanol levels Skin has many of the enzymes that occur in the liver but its metabolising potential is considered too small to be considered for most chemicals (Bando et al., 1997) at an estimated 2% of that of the liver (Pannatier et al., 1978). However, there is some evidence from a more recent ex vivo animal study that repeated dermal exposure may result in increased metabolic activity in the skin (Lockley et al., 2005) . In the second step of ethanol metabolism, AcH is rapidly converted to acetate by the enzyme acetaldehyde dehydrogenase (ALDH).  Acetate formed in the liver following oxidation of ethanol has a high ratio of NADH/NAD+and as a consequence cannot be incorporated into the citric acid cycle. In the final step of ethanol metabolism, acetate produced from the oxidation of AcH is therefore released into the blood and oxidised extra-hepatically to carbon dioxide and water by peripheral tissues.

In a human volunteer study using 24 men and women, it was established that exposures of up to 2000ppm of ethanol for periods of up to 4 hours do not saturate metabolism and that elimination kinetics are first order. A linear relationship was established between exposure concentration and resultant blood ethanol concentrations, leading to the prediction that the a maximum blood ethanol concentration of 2.9mg/l results from an (indefinite) exposure to 1000ppm of ethanol (Seeber, 1994).

Work was carried out with a PBPK model to assess the effects of polymorphism in key enzymes. A review of the literature was conducted to determine the quantitative information available on the changes in the metabolic parameters (Km, Vmax) with polymorphisms in the ADH pathway. The results of the review indicated that while multiple ADH isozymes are present (types), only two ADH genes (ADH2and ADH3) exhibit polymorphisms with ADH2genetic variability having a greater impact on metabolic rate. Using metabolic parameter values for combinations of these different polymorphic forms in the model showed that Cmax and AUC for blood ethanol could vary by a factor of 2 to 4. This suggests that normally used assessment factors for intraspecies variation should be sufficient to account for variations in the ability of individuals to metabolise ethanol through polymorphic forms of key metabolising enzymes (Gentry, 2010).

In a toxicokinetic study designed to produce data to both validate and refine an existing PBPK model, Long-Evans pregnant and non-pregnant rats were exposed to vapour concentrations of ethanol of 5000, 10000 and 21000ppm. At the two lower concentrations, the blood ethanol concentrations rapidly reached a plateau of ~1.5 and 5mg/dL respectively, indicating that metabolism was not saturated at these exposure concentrations. However, at 21000ppm, there was a continuous increase in concentration over the 6 hour exposure period, reaching a final value of ~220mg/dL. (Martin, 2014).

ELIMINATION

The majority of absorbed ethanol is eliminated from the body by metabolism (95 –-98 %; Norberg et al., 2003); the process has limited capacity but this is extremely unlikely to be overwhelmed at the blood ethanol concentrations estimated to result from occupational exposure to ethanol.. The maximum amount of ethanol that can be metabolised per hour has been estimated to be between 83 – 127 mg/kg/hr, or 8 – 9 g ethanol/hr. Elimination rates can be influenced by both environmental and genetic factors leading to intraspecies variation in rates (Jones, 1984). A small concentration of ethanol (2 – 5 %) is also eliminated unmetabolised in breath, urine and sweat (Holford, 1987; Norberg et al., 2003). This conclusion is supported by a study which both contained original data and the results of other published studies, human volunteers were given varying oral doses of ethanol and the elimination rate followed by measuring decaying blood ethanol concentration. Ethanol was found to be rapidly eliminated with a typical elimination rate constant of 11 -15mg/dl/hr over the range of doses examined (0.5 -0.8g/kg). It was noted that the elimination rate increase slightly but significantly with dose and that the range of rate constants varied by a factor of 3 from 8 -13mg/dl/hr in the relatively large number of subjects studied. The half life for elimination in rats following a 1g/kg dose was ~2hrs. Another study examined the concentration-time profiles of ethanol in capillary blood for 21 fasted men after ingesting 0.68g/kg of ethanol. The concentration-time profiles varied significantly between subjects, particularly in peak levels, however peak ethanol concentrations were close the the maximum predicted assuming 100% absorption and instant distribution in body fluids. The rate of elimination was found to be 85mg/kg/hr +/-6.4 with complete disappearance from the blood 494mins +/-38 (8.2hrs) after ingestion. In conclusion, elimination is constant and ethanol taken in at rates below the limiting rate of elimination are not likely to lead to any accumulation of ethanol in the blood.

Blood ethanol concentrations were estimated from breath analysis of a healthy male volunteer over time after consumption of 5 g ethanol diluted in orange juice. The elimination of exogenous ethanol took just over 2 hr, with a half-life of about 16 minutes. Expired alveolar air, from two men and one woman who had consumed between 0.26 and 0.6 g ethanol/kg bw in five separate experiments, was analysed for ethanol concentrations, in an attempt to determine the rate of ethanol disappearance from the human body. The rate of disappearance of ethanol was linear down to levels of ethanol in the serum of 100 mg/L, or in some subjects as low as 50 mg/L. Below this level, it was determined that the rate of disappearance, in mg/L serum/hr, may be calculated as the product of the concentration of ethanol in serum (mg/L) by the constanct 1.64.

A kinetics study using SD rats established an elimination rate of 0.193mg/ml/hr in the first four hours following a 1g/kg dose of ethanol (Asukara, 2014).

In a case study, a 15 year of girl consumed a bottle of tequila sufficient to provide a resultant dose of 4.2 -4.8g/kg. The elimination of ethanol in the post-absorptive phase remained zero-order at a rate of 26.3 mg/dL/ h (5.7 mmol/L/h) with a Pearson’s correlation coefficient (R2) of 0.9968 (p < 0.01). There was no evidence of acute induction in metabolism although pharmacodynamic tolerance likely occurred. Even at very high ethanol concentrations in ethanol naive subjects, elimination of ethanol follows a zero-order toxicokinetic model.

In a toxicokinetic study designed to produce data to both validate and refine an existing PBPK model, Long-Evans pregnant and non-pregnant rats were exposed to vapour concentrations of ethanol of 5000, 10000 and 21000ppm. Following cessation of exposure, blood ethanol concentrations rapidly diminished. At 5000 and 10000ppm exposure, they become undetectable within 30 mins. At the higher concentration, they decayed approximately linearly and the BEC would be down to zero in approximately 2 hours (Martin, 2014).

A physiologically based pharmacokinetic (PBPK) model was developed and applied to a metabolic series

approach for the ethyl series (i.e., ethyl acetate, ethanol, acetaldehyde, and acetate). Existing in vivo pharmacokinetic

studies of ethyl acetate and ethanol were conducted in rats following IV and inhalation exposure was used along with new data where the respiratory bioavailability of ethyl acetate and ethanol were estimated from closed chamber inhalation studies and measured ventilation rates. Regardless of route, ethyl acetate was rapidly converted to ethanol. Blood concentrations of ethyl acetate suggested linear kinetics across blood concentrations from 0.1 to 10 mM ethyl acetate and 0.01e0.8 mM ethanol. Metabolic parameters were optimized and evaluated based on available pharmacokinetic data and a sensitivity analysis performed. The resulting ethyl series model reproduces blood ethyl acetate and ethanol kinetics following IV administration and inhalation exposure in rats, and blood ethanol kinetics following inhalation exposure to ethanol in human (Crowell, 2015).

DISCUSSION ON DERMAL ABSORPTION RATE

In a study to assess the skin penetration potential of ethanol, an in vitro study was carried out using excised pig skin and radiolabelled ethanol. Ethanol penetration was greater under occlusive conditions than non-occlusive conditions, as might be expected. Absorption rates were around 21% and 1% of applied doses respectively. In a study to help understand the skin penetration potential of ethanol, an in vitro study was carried out to assess the evaporation rate of radiolabelled ethanol from excised pig skin. The evaporation half life was found to be 11.7seconds, suggesting that systemic doses of ethanol resulting from skin absorption following single exposures to ethanol will, under practical conditions, be very low due to rapid evaporation. In a study to assess the potential for skin uptake of ethanol from the use of personal care products, human volunteers applied ethanol over their whole bodies delivered from an aerosol can. The mean dose of ethanol applied was just under 10g per subject. Ethanol could not be unequivocally detected in the blood of any of the 16 volunteers in the hour immediately after application (Pendlington, 2001).

An in vitro system was used to assess the penetration of radiolabelled ethanol through excised, full thickness, guinea-pig skin. Less than 1% of the applied dose of ethanol penetrated the "uncovered" skin over a period of 19 hours, and there did not appear to be an increase in penetration with increasing dose volume. When the test system was "occluded" using Parafilm, a polyester Gel Bond film, or plastic Hill Top chambers, the penetration was significantly enhanced (Gummer, 1986).

The absorption of ethanol was measured in the blood of twelve volunteers following repeated application of three different ethanol based hand disinfectants containing different concentrations of ethanol (95, 85, and 55%, respectively). Two exposure regimes were evaluated, mimicking worst case hygienic and surgical hand disinfection. Blood was sampled prior to, and 2.5, 5, 10, 20, 30, 60, and 90 minutes after the last hygienic disinfection. After the last application the median absorbed ethanol in the blood increased gradually and peaked after 30 minutes. The amount of absorbed ethanol was calculated at 2.3, 1.1, and 0.9% with hand rubs A, B, and C, respectively in the hygienic disinfectant simulation and 0.7, 1.1, and 0.5% with hand rubs A, B, and C, respectively in the surgical hand disinfectant simulation. In another volunteer study of 26 healthcare workers, absorption of ethanol by inhalation or dermal absorption was not detected from urine or blood analysis. Ethanol vapour was detected in the workplace at peaks up to 142mg/m3 (Kramer, 2007).

In a study to examine the potential for systemic exposure to ethanol through the use of ethanol based skin sanitiser, male volunteers were exposed occlusively to ethanol on their backs at ~80mg/cm2 or 200mg.kg for a period of 10 minutes. Some transient erythema was seen but there was no evidence of any dermal absorption of ethanol during the 1 hour observation period based on blood analysis results (Kirschner, 2009).

In a volunteer study of 26 healthcare workers, absorption of ethanol by inhalation or dermal absorption was not detected from urine or blood analysis. Ethanol vapour was detected in the workplace at peaks up to 142mg/m3 (Hautemaniere, 2013).

References not included elsewhere:

Bando H, Mohri S, Yamashita F, Takakura Y, Hashida M. (1997) Effects of skin metabolism on percutaneous penetration of lipophilic drugs. J Pharm Sci. 86(6):759-61.

Bevan RJ1, Slack RJ, Holmes P, Levy LS (2009). An assessment of potential cancer risk following occupational exposure to ethanol. J Toxicol Environ Health B Crit Rev. 12(3):188-205.

Crabb DW, Bosron WF, Li TK (1987) Ethanol metabolism. Pharmac Ther, 34, 59-73.

Holford NH (1987) Clinical pharmacokinetics of ethanol. Clin Pharmacokinet. 13(5):273-92.

Jones AW (1984). Interindividual variations in the disposition and metabolism of ethanol in healthy men. Alcohol. 1(5):385-91.

Norberg A, Jones AW, Hahn RG, Gabrielsson JL. (2003) Role of variability in explaining ethanol pharmacokinetics: research and forensic applications. Clin Pharmacokinet, 42(1):1-31

Pannatier A, Jenner P, Testa B, Etter JC (1978). The skin as a drug-metabolizing organ. Drug Metab Rev. 8(2):319-43.

Pastino GM, Flynn EJ, Sultatos LG (2000) Genetic polymorphisms in ethanol metabolism: issues and goals for physiologically based pharmacokinetic modeling. Drug Chem Toxicol. 23(1):179-201.