Chemical Exposure Limit Calculator
Calculate Time-Weighted Average (TWA) exposure, Hazard Quotient (HQ), and determine if workplace chemical exposure is within safe limits based on OSHA/NIOSH/ACGIH standards.
Exposure Measurement Inputs
Occupational Exposure Limit
Extended Shift Adjustment (Optional)
Formulas Used
Time-Weighted Average (TWA):
TWA = Σ(Cᵢ × Tᵢ) / Tshift
Where Cᵢ = concentration during period i, Tᵢ = duration of period i (hours), Tshift = total shift duration (hours).
Hazard Quotient (HQ):
HQ = TWA / OEL
HQ < 1 → Acceptable exposure; HQ ≥ 1 → Exceeds limit (action required).
Brief & Scala Extended Shift Adjustment:
RF = (8/h) × (24 − h) / 16
Adjusted OEL = OEL × RF
Where h = hours worked per shift. Valid for shifts between 8 and 12 hours.
Percent of OEL:
% OEL = (TWA / OEL) × 100
Assumptions & References
- TWA is calculated over the actual shift duration entered, not normalized to 8 hours unless OEL adjustment is applied.
- OSHA PEL values are based on 29 CFR 1910.1000 Table Z-1 (8-hr TWA basis).
- NIOSH REL values are based on a 10-hour workday, 40-hour workweek.
- ACGIH TLV-TWA values are based on an 8-hour workday, 40-hour workweek.
- The Brief & Scala model is a widely accepted pharmacokinetic model for adjusting OELs for non-standard shifts (AIHA, 1991).
- Brief & Scala adjustment is only valid for shifts between 8 and 12 hours; results outside this range are flagged.
- This calculator does not account for skin absorption, mixture toxicity, or STEL (Short-Term Exposure Limits).
- Results are for informational purposes only. Consult a Certified Industrial Hygienist (CIH) for regulatory compliance.
- References: OSHA 29 CFR 1910.1000; NIOSH Pocket Guide to Chemical Hazards; ACGIH TLVs and BEIs; Brief RS & Scala RA (1975), AIHA Journal.