IH Unit Converter — Industrial Hygiene

What's covered

This converter specializes in industrial hygiene and ventilation engineering units, the ones a generic converter skips or gets wrong.

Pressure — Pa, hPa, kPa, MPa, mbar, bar, atm, psi/psia/psig, mm Hg, in Hg, in H₂O (ACGIH convention at 60 °F), mm H₂O, cm H₂O. Essential for duct static pressure, manometry, and portable instrument readings.
Volumetric flow — L/min, m³/min, m³/h, m³/s, cfm, scfm. Use in ventilation design, local exhaust, and industrial process specifications. Advanced mode converts between scfm and acfm using actual T and P.
Air velocity — m/s, m/min, fpm (ft/min), ft/s. Duct traverses, capture velocity, face velocity measurements — all in one place.
Aerosol concentration — mg/m³ ↔ µg/m³ for mass-based concentrations; fibres/cc ↔ fibres/L for fibre counting. Cross-group conversion (fibres to mg/m³) is blocked — those units are incompatible without additional data.

Why "in H₂O" matters in ventilation

Inches of water column (in H₂O, also written "in. w.g." or "in. w.c.") is the dominant unit for low-pressure measurements in North American HVAC and industrial ventilation. Duct static pressure, fan curve ratings, and portable manometer readings are routinely expressed in in H₂O. The exact conversion factor depends on the temperature at which water density is referenced.

ACGIH and ASHRAE use the 60 °F reference: 1 in H₂O = 249.089 Pa. Some older literature uses 68 °F (20 °C), giving 249.098 Pa — a difference of less than 0.004%, negligible in practice. This tool uses the 60 °F / ACGIH convention, consistent with the Industrial Ventilation manual.

Practical example: a hood static pressure of 2 in H₂O = 498.2 Pa = 4.98 mbar. When reporting in a professional document, specify your reference temperature to avoid ambiguity.

psia vs psig, the silent error

psi alone is ambiguous. It can mean psia (pounds per square inch absolute) or psig (pounds per square inch gauge). The difference is 14.696 psi — atmospheric pressure at sea level. A compressed-air line reading 90 psig is actually at 104.696 psia. Confusing the two produces errors of 14–15 psi, which can be significant in HVAC calculations, process safety, and respirator selection.

When converting psi in this tool, use the psia/psig toggle to declare your reference. The converter applies the 14.696 psi (101 325 Pa) offset automatically. Gauge pressure is used when reading directly from a pressure gauge; absolute pressure is needed for thermodynamic and gas-law calculations.

When to Use the Industrial Hygiene Units Converter

Industrial hygiene (IH) involves measuring and controlling workplace hazards — chemical, physical, and biological. Unit conversions are critical when: comparing exposure measurements to OSHA PELs or ACGIH TLVs (which use different unit systems), converting between mg/m³ and ppm for chemical exposure reports, translating noise measurements between dB(A) and Pa², and reconciling data from instruments calibrated in different unit systems. Errors in IH unit conversion can lead to incorrect risk assessment and inadequate worker protection.

Key Industrial Hygiene Unit Systems

Key unit domains in industrial hygiene:

Chemical concentration — ppm (parts per million by volume), mg/m³, µg/m³, ppb. Conversion requires molecular weight: mg/m³ = ppm × MW / 24.45 (at 25°C, 1 atm).
Noise — dB SPL (sound pressure level), dB(A) (A-weighted for hearing), Pa (Pascals), µPa. Reference: 0 dB SPL = 20 µPa (threshold of hearing).
Radiation — mSv (millisieverts), mR/h (milliroentgens per hour), µGy (microgray), Bq (becquerel).
Heat stress — °C, °F, WBGT (Wet Bulb Globe Temperature), relative humidity %.
Illuminance — lux (lx), foot-candles (fc). 1 fc = 10.764 lux.

Frequently Asked Questions

What's the difference between scfm and cfm?

cfm (cubic feet per minute) is the actual volumetric flow at the local conditions (temperature, pressure). scfm (standard cfm) normalises the flow to reference conditions — typically 70 °F and 14.7 psia (ASHRAE standard). At standard conditions, scfm and cfm are numerically identical. At other temperatures or elevations, you need the ideal-gas correction: V_std = V_actual × (T_std / T_act) × (P_act / P_std). This matters in high-altitude sites and in processes with significant temperature differences.

Why is in H₂O measured at 60 °F?

The 60 °F (15.6 °C) reference was adopted by ACGIH in the Industrial Ventilation manual and by ASHRAE for HVAC measurements. At that temperature, water density is 999.01 kg/m³, giving the 249.089 Pa/in H₂O factor. The choice is historical and practical, it reflects the approximate temperature of water in many measurement environments. ACGIH has used this value consistently since the 1950s. If your instrument uses a different reference, you should note it in your field records.

Is this different from the regular Unit Converter?

Yes. The general Unit Converter covers length, weight, volume, area, speed, energy, and data — everyday categories. This tool covers IH-specific units that require specialized handling: in H₂O (temperature-referenced), psia/psig disambiguation, scfm/acfm correction for non-standard conditions, and fibre counting units. The general converter also does not separate psi absolute from gauge.

What's a fibre/cc and when is it used?

Fibres per cubic centimetre (f/cc) is the standard unit for expressing airborne fibre concentrations — particularly for asbestos and man-made mineral fibres (MMMF). It is used in occupational exposure limits (Quebec: RSST Annex I, OSHA PEL), and is the unit reported by phase-contrast optical microscopy (PCME) and transmission electron microscopy (TEM). 1 f/cc = 1000 f/L. Fibre counts cannot be converted to mass concentrations (mg/m³) without additional data on fibre dimensions and density.

Why can't I convert fibres/cc to mg/m³?

Fibres/cc is a count-based unit (number of fibres per volume). mg/m³ is a mass-based unit. Converting between them requires knowing the average fibre length, diameter, and bulk density — data that vary by material type and cannot be assumed. The tool blocks this conversion to prevent silent errors in professional reports.

What is the difference between ppm and mg/m³ for chemical exposure limits?

Both express airborne chemical concentration, but ppm is volume-based (independent of molecular weight) while mg/m³ is mass-based. OSHA PELs and ACGIH TLVs often publish both. Conversion: mg/m³ = ppm × MW / 24.45 (at 25°C, 1 atm), where MW is molecular weight in g/mol and 24.45 is the molar volume of an ideal gas. Example: 1 ppm of toluene (MW=92.14) = 92.14/24.45 = 3.77 mg/m³.

What are OSHA PELs and ACGIH TLVs, and which should I use?

OSHA PELs (Permissible Exposure Limits) are legal maximum limits under US federal law — mandatory minimums. ACGIH TLVs (Threshold Limit Values) are health-based guidelines published annually by occupational health scientists — typically more conservative and more up-to-date than PELs, many of which haven't been updated since 1971. Best practice: use TLVs as your target exposure goal and PELs as the legal compliance floor. In Canada, provincial OELs (Occupational Exposure Limits) are generally based on ACGIH TLVs.