In this episode, Dr. Ayers explains how to convert airborne contaminant concentrations measured in mg/m³ into parts per million (PPM)—a calculation safety professionals often need when comparing sampling results to OSHA or ACGIH exposure limits.
The episode focuses on understanding the conversion formula, when to use it, and how to automate the calculation for consistent, error‑free reporting.
mg/m³ measures mass per volume
PPM measures volume per volume Because gases expand and contract with temperature and molecular weight, you can’t convert between them without adjusting for chemistry and conditions.
Dr. Ayers walks through the standard industrial hygiene formula:
PPM=mg/m3⋅24.45Molecular Weight\text{PPM} = \frac{\text{mg/m}^3 \cdot 24.45}{\text{Molecular Weight}}
Where:
24.45 is the molar volume of air at 25°C and 1 atm
Molecular Weight is specific to the chemical sampled
This formula allows you to convert any mg/m³ result into PPM for comparison with exposure limits.
Comparing mg/m³ sampling results to PPM‑based OSHA PELs
Aligning lab results with ACGIH TLVs
Standardizing data across different sampling methods
Communicating results to supervisors and employees in a familiar unit
Dr. Ayers discusses:
Setting up a spreadsheet or automated system
Pre‑loading molecular weights
Reducing transcription errors
Making conversions repeatable and audit‑ready
This is especially useful for safety teams handling multiple chemicals.
Always check whether the exposure limit is in PPM or mg/m³—they are not interchangeable.
Know the molecular weight of the chemical you’re evaluating.
Use the 24.45 constant for standard conditions unless you have reason to adjust.
Automate conversions to reduce mistakes and speed up reporting.
