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Linear mixed models were used to analyze data compiled from international sources to simultaneously estimate the fixed effects, associated with process characteristics and sampling regimen, and the variance components, associated with the random effects. The fixed effects explained 55% and 49% of variation in TP and Mn exposures, respectively. The country, industry/trade, ventilation condition, type of work/welding process, and material employed appeared to be the major factors affecting exposures to TP and Mn. Measurements in the U.S. were generally higher than those in other countries. Exposure to TP was 64% higher in enclosed spaces and 42% lower with local exhaust ventilation, was higher among boiler makers, and was higher when a mild-steel base metal was used. Exposure to Mn was 318% higher in enclosed spaces and 67% lower when local exhaust ventilation was present. The measured Mn air concentration was significantly related to the composition of the consumables, but not to the base metal. Resistance welding produced significantly lower TP and Mn exposures compared to other welding processes. After controlling for fixed effects, variance components between groups and between individual workers within a group were reduced by 89% and 57% for TP, and 75% and 63% for Mn, respectively. The within-worker variance component in Mn exposure was three times higher than that of TP, indicating that day-to-day and within-day variations in TP and Mn exposures were influenced by different factors that were not captured equally well by the mixed models for these two contaminants. Interestingly, exposures to TP and Mn had not changed over the 40 years of observation. The estimated probabilities of exceeding occupational exposure limits were very high (generally much greater than 10%) for both agents. Welding exposures to TP and Mn vary considerably across the world and across occupational groups. Exposures to both contaminants have been and continue to be unacceptably high in m

Linear mixed models were used to analyze data compiled from international sources to simultaneously estimate the fixed effects, associated with process characteristics and sampling regimen, and the variance components, associated with the random effects. The fixed effects explained 55% and 49% of variation in TP and Mn exposures, respectively. The country, industry/trade, ventilation condition, type of work/welding process, and material employed appeared to be the major factors affecting exposures to TP and Mn. Measurements in the U.S. were generally higher than those in other countries. Exposure to TP was 64% higher in enclosed spaces and 42% lower with local exhaust ventilation, was higher among boiler makers, and was higher when a mild-steel base metal was used. Exposure to Mn was 318% higher in enclosed spaces and 67% lower when local exhaust ventilation was present. The measured Mn air concentration was significantly related to the composition of the consumables, but not to the base metal. Resistance welding produced significantly lower TP and Mn exposures compared to other welding processes. After controlling for fixed effects, variance components between groups and between individual workers within a group were reduced by 89% and 57% for TP, and 75% and 63% for Mn, respectively. The within-worker variance component in Mn exposure was three times higher than that of TP, indicating that day-to-day and within-day variations in TP and Mn exposures were influenced by different factors that were not captured equally well by the mixed models for these two contaminants. Interestingly, exposures to TP and Mn had not changed over the 40 years of observation. The estimated probabilities of exceeding occupational exposure limits were very high (generally much greater than 10%) for both agents. Welding exposures to TP and Mn vary considerably across the world and across occupational groups. Exposures to both contaminants have been and continue to be unacceptably high in m