The respiratory tract is the main route of exposure. Xylene is rapidly absorbed and very soluble in blood and tissues (Astrand et al., 1978). Pulmonary retention of xylene is approximately 60-65% (Sedivec and Flek, 1976; Astrand et al., 1978; Riihimäki et al., 1979; Riihimäki and Savolainen, 1980; Liira et al., 1988; Laine et al., 1993). Uptake remains constant even after tissue equilibrium is reached (Riihimäki and Savolainen, 1980). The magnitude of uptake has been found to correlate with the amount of body fat (Engström and Bjurström, 1978).

Uptake of xylene is increased with exercise (Astrand et al., 1978; Gamberale et al., 1978; Riihimäki and Savolainen, 1980; Savolainen et al., 1980; Laine et al., 1993) due to increased respiration and heart rate. Increased uptake of 30-50% has been reported during exercise (Riihimäki and Savolainen, 1980).

The concentration of xylene in the air correlates with the concentration of xylene in the blood (Krämer et al., 1999) and the urinary concentration of methylhippuric acids (Engström et al., 1978; Lundberg and Sollenberg, 1986; Kawai et al., 1991; Krämer et al., 1999). This is true whether the isomers are considered separately or together (Kawai et al., 1991). Fluctuations in the air concentration of xylene result in relatively rapid changes in the tissue, and particularly the blood, concentration (Riihimäki and Savolainen, 1980; Riihimäki and Hänninen, 1987); this has implications for biological monitoring.

Xylene is absorbed through the skin. In a volunteer study where both hands were immersed in xylene for 20 minutes, xylene was detectable in expired air about 10 minutes after the start of exposure. The peak concentration occurred at 30 minutes (i.e., 10 minutes after exposure ceased). Use of a barrier cream did not influence the dermal absorption of xylene (Lauwreys et al., 1978). In a similar experiment where volunteers placed a hand in m-xylene for 15 minutes the blood concentration was fifty times greater in the sample from the exposed arm than from the non-exposed arm. This difference persisted for five hours suggesting a reservoir of xylene and continued absorption (Engström et al., 1977).

Xylene is readily absorbed from the gastrointestinal tract. Death has occurred from ingestion of xylene (Abu Al Ragheb et al., 1986).

Toxicology of Solvents Distribution

Xylene is taken up quickly by the blood and then rapidly redistributed to tissues. Well perfused tissues such as the brain rapidly reach an equilibrium xylene concentration. The concentration in the brain probably closely follows that in the blood (Riihimaki and Savolainen, 1980).

In prolonged exposure xylene accumulates in adipose tissue. However, the distribution is low compared to other solvents; only about 4-8% of absorbed xylene is retained in fat (Engstrom and Bjurstrom, 1978; Engstrom and Riihimaki, 1979). In a volunteer study, distribution to adipose tissue was affected by exposure pattern. When exercise was performed during exposure a proportionally greater distribution to adipose tissue occurred (Engstrom and Riihimaki, 1979).

In a suicidal case of xylene ingestion the highest concentration was found in the duodenum (3,300 mg/dl), followed by the gastric contents (880 mg/dl) and the blood (11 mg/dl). Postmortem examination revealed no significant changes except pulmonary oedema. The time between ingestion and death was not stated (Abu Al Ragheb et al., 1986).

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