Letters to the Editor
To the Editor:
Chang et al.1 presented results from a small study to assess the relative importance of inhalation and dermal exposure to solvents among a group of shipyard spray painters. Air samples were obtained inside and outside respirators using 3 M Organic Vapor Monitors (3M, St. Paul, MN) or Dräger Orsa 5 diffusive monitors (Drägerwerk, AG, Lübeck, Germany). Dermal exposure was assessed using two layers of charcoal cloth fixed to an impervious backing that was attached to the skin of workers, as originally recommended by Cohen and Popendorf.2 Chang et al. measured ethyl benzene and xylene exposure from the paint solvent.
The authors assume that the air samplers only measured the concentration of solvent vapors and that the charcoal pads only measured dermal exposure. However, all of the samplers they used will respond to solvents in the air as well as from contact with bulk paint or splashed paint droplets. In particular, the charcoal pads are effective at collecting solvent vapors from the air and there is no indication that the authors corrected their measurements for this bias. In research carried out in my own laboratory with toluene we showed that a charcoal pad had a vapor uptake rate of 3600 cm3/min.3 Assuming a similar uptake rate for charcoal cloth with xylene would result in about 15 mg being sampled from vapor in the air, based on an air concentration of 29.4 ppm, the average they measured outside the respirator, and adjusting for the smaller sampler area used by Chang et al. compared with our own work. This value is about 5% of the average solvent mass they measured “inside blocks” and about 30% of the mass measured “outside blocks.”
Chang et al. also noted a strong correlation between the results of their inhalation and dermal measurements, and between results from dermal measurement at various body parts, both of which are indicative that the dermal samplers were unusually strongly influenced by the air vapor concentrations of the solvents or paint aerosol. For example, Hughson and Aitken4 showed dermal exposure of spray painters was greatest on their hands and only sporadically affected other body parts.
Any solvent exposure from paint droplets that land on the skin is likely to evaporate before it is taken up through the skin, with only a small proportion diffusing through the stratum corneum and being available for systemic distribution. However, solvent in droplets that land on the charcoal pads will be almost completely adsorbed onto the sampler. This results in an overestimation of actual dermal exposure, which is a previously reported limitation of this dermal sampling methodology. Based on the Chang et al. data of about 20 mg/cm2 xylene there would have been about 0.3 kg whole body dermal exposure to this solvent.
As well as external exposure measures Chang et al. undertook biological monitoring of the levels of methyl hippuric acid (MHA) and mandelic acid in urine. The levels that they measured were low: eg, average increase over a work shift of MHA was 61.4 mg/g creatinine. Data from Jacobson and McLean5 suggest that this level of MHA increase would be consistent with air exposure to about 4 ppm of xylene, which is similar to the levels Chang et al. measured inside the respirators worn by the painters (ie, average 1.2 ppm). These data suggest that the urinary metabolite exposure may be a result of the painters' inhalation exposure and that the heavy dermal exposure contributed the equivalent of no more that 2 to 3 ppm of inhaled vapor for xylene and a similar level for ethyl benzene.
As we have proposed, the best way to measure dermal exposure to volatile agents is using a sampling device designed to mimic the process of uptake through the skin, ie, a diffusion process with a membrane limiting the rate of uptake and allowing evaporation of the agent before passing through the stratum corneum.3 Dermal exposure data for solvents obtained using simple charcoal cloth samplers are likely to be unreliable and overestimate the “biologically relevant” dermal exposure. It seems unlikely, in my opinion, that dermal exposure to solvents made an important contribution to the solvent exposure of the shipyard workers in this study.
John W. Cherrie, PhD, BSc
Institute of Occupational Medicine
Research Avenue North
Edinburgh, United Kingdom
1. Chang FK, Chen ML, Cheng SF, Shih TS, Mao IF. Dermal absorption of solvents as a major source of exposure among shipyard spray painters. J Occup Environ Med
2. Cohen BS, Popendorf W. A method for monitoring dermal exposure to volatile chemicals. Am Ind Hyg Assoc J
3. Lindsay FE, Semple S, Robertson A, Cherrie JW. Development of a biologically relevant dermal sampler. Ann Occup Hyg
4. Hughson GW, Aitken RJ. Determination of dermal exposures during mixing, spraying and wiping activities. Ann Occup Hyg
5. Jacobson GA, McLean S. Biological monitoring of low level occupational xylene exposure and the role of recent exposure. Ann Occup Hyg
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