Ventilation Dilution Calculator

This calculator applies the classic industrial-hygiene dilution ventilation equation, Q = (G × K × 10⁶) / (C_limit − C_supply), to figure out how much clean air must be supplied to keep an airborne contaminant below its exposure limit. The generation rate you enter in mg/min is first converted to a volumetric rate (cubic feet per minute of pure vapor) using the ideal gas law at standard conditions — 24.45 liters per mole at 25°C — divided by the contaminant's molecular weight and the mg-to-gram and liter-to-cubic-foot conversions. That vapor generation rate is then multiplied by a safety/mixing factor K (typically 3 for good mixing up to 10 for poor mixing or highly toxic substances) and by a million to convert to parts-per-million terms, then divided by the effective concentration limit — the TLV or PEL minus whatever background concentration is already present in the supply air. The result is Required Ventilation in CFM, also expressed as air changes per hour for your room volume and as a dilution ratio (clean air volume needed per unit of contaminant vapor).

The K factor is doing a lot of work here and is a judgment call, not a measured quantity: real rooms never mix perfectly, so this general-ventilation approach is only appropriate when contaminant sources are diffuse and workers aren't stationed right next to the source. If air changes needed climbs above roughly 60 ACH, dilution ventilation is usually impractical and local exhaust at the source is the better engineering control — dilution is a last resort in industrial hygiene practice, precisely because it dilutes contaminants into air people are still breathing rather than capturing them at the source. This tool also assumes a single, well-characterized contaminant with steady generation; it doesn't account for multiple simultaneous contaminants, intermittent releases, or short-term excursions above the limit, all of which need separate analysis.