Topical absorption and toxicity studies of jet fuel hydrocarbons in skin (2004)

Kerosene-based fuels have been used for many decades. Over 2 million military and civilian personnel each year are occupationally exposed to various jet fuel mixtures. Dermatitis is one of the major health concerns associated with these exposures. In the past, seperate absorption and toxicity studies have been conducted to find the etiology of such skin disorders. There was a need for integrated absorption and toxicity studies to define the causative constituents of jet fuel responsible for skin irritation. The focus of this thesis was to study the percutaneous absorption and to identify the hydrocarbons (HC) causing irritation in jet fuels so that preventive measures could be taken in the future. \n\n\tThe initial study was conducted to understand the possible mechanism for additive interactions on hydrocarbon absorption/disposition in silastic, porcine skin and isolated perfused porcine skin flap (IPPSF) models. The influence of JP-8 (100) additives (MDA, BHT, 8Q405) on the dermal kinetics of 14C-naphthalene and 14C/3H-dodecane as markers of HC absorption was evaluated. This study indicated that individual and combination of additives influenced marker disposition in different membranes. MDA was a significant suppressor while BHT was a significant enhancer of naphthalene absorption in IPPSF. The 8Q405 significantly reduced naphthalene content in dosed silastic and skin indicating a direct interaction between additive and marker HC. Similarly, the individual MDA and BHT significantly retained naphthalene in the stratum corneum of porcine skin, but the combination of both of these additives statistically decreased the marker retention in the stratum corneum suggesting a potential biological interaction. This study concluded that all components of a chemical mixture should be assessed since the effects of single components administered alone or as pairs may be confounded when all are present in the complete mixture. However, this study indicated that the marker HC absorption was similar across JP-8 and JP-8 (100) due to the opposite effects of MDA and BHT on HC absorption. \n\nThe remaining studies were focused on neat HC absorption and toxicity potential in pig skin. There were no published reports regarding dose-related percutaneous absorption of jet fuel HC that are crucial for risk assessment studies. Three dosing mixtures (1X, 2X, and 5X) comprising 5 aliphatic (C11-C15) and 2 aromatic (naphthalene and dimethyl naphthalene (DMN)) HC were dosed using in vitro porcine skin flow-through diffusion cells with hexadecane as the diluent. Perfusate samples were analyzed with gas chromatography flame ionization detector (GC-FID) using a headspace solid phase micro-extraction (SPME) fiber technique. Absorption parameters were estimated and quantitative structure permeability relationship model was constructed to predict the permeability of unknown jet fuel HC using their known physico-chemical properties. This study suggested a dose related increase in percutaneous absorption of aromatic HC. This dose related increase in absorption of HC focuses attention to occupational settings where workers are exposed to jet fuels on daily basis. In order to assess the effects of jet fuel pre-exposure on subsequent dermal absorption of HC, pigs were exposed to JP-8 jet fuel soaked cotton fabrics for 1-day and with repeated daily application for 4-days to mimic the real occupational scenario. A dosing mixture comprising 8 aliphatic (C9-C16) and 6 aromatic HC (ethyl benzene, o-xylene, trimethyl benzene (TMB), cyclohexyl benzene (CHB), naphthalene and DMN) was dosed to in vitro flow-through diffusion cells containing 1 and 4-day JP-8 pre-exposed and control skin using water + ethanol (50:50) as diluent. Perfusate samples were analyzed with GC-FID and SPME fiber technique. The data revealed that there was 2-3 fold and 3-4 fold increase in percutaneous absorption of short chain aliphatic and most of aromatic HC through 1-day and 4-day JP-8 pre-exposed skin, respectively. Three aromatic HC (naphthalene, CHB and DMN) were found to have persistent skin retention in 4-day JP-8 pre-exposures. Stratum corneum studies with Fourier Transform Infrared (FTIR) spectroscopy suggested that lipid extraction might be the primary mechanism for this increase in hydrocarbon absorption through JP-8 pre-exposed skin. These studies indicated that the single dose absorption data from naieve skin for jet fuel HC may not be optimal to predict the toxic potential for repeated exposures. \n\nFinally, in vivo studies with the individual hydrocarbons (HC) of jet fuel were conducted to identify the causative agents in fuel induced skin irritation. In order to mimic occupational scenarios, cotton fabrics soaked with 300&#956;l of JP-8 and 14 different aliphatic and aromatic HC were placed on back of pigs for 1-day and with repeated daily exposures for 4-days. The 1-day in vivo HC exposures revealed no erythema but significant redness was observed in 4-day tridecane, tetradecane, pentadecane and JP-8 exposed sites. The aromatic HC did not produce any macroscopic lesions in 1 or 4-day in vivo exposures. Epidermal thickening and epidermal cellular layers were significantly (P < 0.05) different in tridecane, tetradecane, pentadecane and JP-8 treated sites as compared to control sites. No significant differences were observed in aromatic HC exposed sites. Subcorneal micro abscesses filled with inflammatory cells were observed with most of long chain aliphatic HC and JP-8 in 4-day in vivo exposures. Ultrastructural studies depicted that jet fuel HC induced cleft formation in the intercellular lipid lamellar bilayers. The degree of damage to the stratum corneum and epidermis was proportional to the length of in vivo HC exposures. This study indicated that the aliphatic HC are the main source of jet fuel caused irritation. A comparison of absorption and toxicity data for aliphatic HC revealed that toxic point might be in the transition of absorption to retention phases of these HC as tridecane is the last HC that was absorbed through the skin and tetradecane was the first HC to be retained in the skin. Furthermore, the macroscopic and microscopic alterations caused by these two HC in these in vivo studies were similar to the JP-8 exposed animals, suggesting that tridecane and tetradecane might be toxicologically important HC in jet fuel induced skin irritation.