The conversion. At 25 °C and 1 atmosphere one mole of any ideal gas occupies 24.45 litres, so mg/m³ = ppm × molecular weight ÷ 24.45 and ppm = mg/m³ × 24.45 ÷ molecular weight. Expressed as a concentration, 1 ppm is one part in a million by volume, which is 1 mg per cubic metre only when the molecular weight happens to be 24.45 — for carbon monoxide at 28.01 g/mol, 35 ppm is 35 × 28.01 ÷ 24.45 = 40.1 mg/m³, not 35.

Temperature and pressure. Exposure limits are quoted at 25 °C and 760 mmHg. At other conditions the general form is mg/m³ = ppm × MW × P ÷ [62.4 × (273.15 + T °C)], with P in mmHg. The correction matters: a hot confined space or a working fire shifts the molar volume enough that a limit quoted at standard conditions and a reading taken at 60 °C are not the same number, and the same mass of gas is a different ppm at altitude.

Percent LEL. A combustible gas instrument reads percent of the lower explosive limit, not a concentration. ppm at a given %LEL = (%LEL ÷ 100) × LEL % by volume × 10,000. So a gas with a 4 % LEL reading 25 % LEL is at 1 % by volume, or 10,000 ppm. The instrument is telling you how far you are from ignition, not how toxic the atmosphere is — plenty of materials are lethal at a small fraction of their LEL, and plenty of toxic gases have no LEL at all because they do not burn. Never use an LEL reading as a toxicity reading, and never use a PID or colorimetric tube as a flammability reading.

Oxygen. Normal air is 20.9 %. Below 19.5 % is an oxygen-deficient atmosphere by definition and requires supplied air — not a cartridge respirator. Above 23.5 % is oxygen-enriched and materially raises the ignition and burn rate of everything in the space. Between those two, the reading still tells you nothing about what is displacing the oxygen, so treat a normal O₂ figure as a necessary condition for entry, never a sufficient one.

What is deliberately not in this tool. Molecular weights are in the dropdown because they are arithmetic — the sum of atomic masses, and the same everywhere. Exposure limits, IDLH values, PELs, RELs and lower explosive limits are not, because they are regulatory and product-specific, they change, and they vary by jurisdiction and by formulation. Look them up in the current NIOSH Pocket Guide, the applicable SDS and the Emergency Response Guidebook for the specific entry number, and type the LEL in here yourself. A conversion calculator that invented an IDLH would be worse than no calculator.

Other limits. The 24.45 figure assumes ideal gas behaviour, which holds well for the small molecules involved here at ordinary pressures and breaks down for heavy vapours at high pressure. It says nothing about mixtures — the additive formula for mixed exposures is a separate calculation — about particulates, about dermal absorption, or about instruments that are out of calibration, cross-sensitive to another gas, or bumped against their span gas this morning.

Disclaimer. A unit-conversion aid for training and reference. Hazardous materials decision-making must follow the Emergency Response Guidebook, the applicable SDS, your department's hazmat SOPs and the incident commander. Sources: AIHA The Occupational Environment · NIOSH Pocket Guide to Chemical Hazards · ACGIH TLVs and BEIs · DOT Emergency Response Guidebook. Last reviewed: 2026-09-21.