The employer must make sure no employee breathes an airborne concentration of any hazardous substance (supplied to the workplace or generated there) that is higher than the exposure standard, if one exists, for the substance or for any ingredient of it. This is an absolute ceiling rather than a target to be reached as far as practicable. The duty supports section 21 of the Act.
The graph holds this control, the 0 it maps to, and the evidence behind each claim, over MCP and REST.