On this basis, the Secretary of Labor had set the permissible exposure level for workers at 1 ppm (part per million), which he considered the lowest feasible level.是什么意思_On this basis, the Secretary of Labor had set the permissible exposure level for workers at 1 ppm (part per million), which he considered the lowest feasible level.在線(xiàn)翻譯_英語(yǔ)_讀音_用法_例句