We used a simple one-way ANOVA and Bonferroni's post-hoc test for multiple analyses to evaluate differences in metal concentrations in air samples between the three recycling work tasks without stratification by company. We also tested for interactions between companies and work tasks using a univariate ANOVA with an interaction term “company × work task”. If an interaction was indicated (p b 0.1), we studied the difference in air concentrations between work task groups on a company level. This method assumes equal variances; therefore, we used Levene's test of equality of error variances. If this test was significant at the p-level of 0.05, we used the non-parametric Kruskal– Wallis to evaluate work task differences within each company.