The TLV for chemical substances is defined as a concentration in air,tropically for inhalation or skin exposure.Its units are in part per million (ppm) for gases and in milligrams per cubic meter(mg/m3) for particulates such as dust,smoke and mist. The basic formula for converting between ppm and mg/m3 for gases is ppm=(mg/m3)*24.45/ molecular weight.This formula is not applicable to airborne particles.