Exposure limits are written in ppm in one document and in mg/m³ in the next, and detectors read in whichever their maker chose. Pick the gas, or enter a molar mass, and convert either way.
mg/m³ = ppm × M / Vm, where M is the molar mass of the gas in g/mol and Vm is the volume of one mole of gas. At 25 °C and 101.325 kPa, Vm is about 24.45 L/mol, which is the basis most occupational exposure limits use.
ppm is a ratio of volumes and does not change with conditions, but mg/m³ does, because a cubic metre of air holds more gas when it is cold or compressed. Some regulations use 0 °C or 20 °C instead of 25 °C; set the conditions your limit is written at.
The conversion needs one molar mass, so it applies to a single compound. A reading for a group such as total VOC or combustible gas is calibrated against one reference gas, and only converts with that gas in mind.
About 1.15 mg/m³ at 25 °C and normal pressure, from a molar mass of 28.01 g/mol. So 35 ppm is about 40 mg/m³.
They assume different conditions (0, 20 or 25 °C) or round the molar volume differently. Check which basis the limit you are working to is written at.
No. ppm by volume only describes a gas or vapour mixed in air. Dust and aerosols are measured by mass directly, in mg/m³.
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