Twa Calculation

Time Weighted Average (TWA) Calculator

Use this tool to calculate the 8-hour Time Weighted Average (TWA) for chemical or noise exposure levels based on different time intervals throughout a shift.

Calculation Results

Total Exposure (C × T): 0

Total Time Sampled: 0 hours

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8-Hour TWA: 0

function calculateTWA() { var totalCT = 0; var totalT = 0; for (var i = 1; i 0) { totalCT += (c * t); totalT += t; } } if (totalT === 0) { alert("Please enter at least one concentration and duration."); return; } // Standard OSHA TWA is calculated based on an 8-hour shift // Formula: (C1T1 + C2T2 + … CnTn) / 8 var twaValue = totalCT / 8; document.getElementById('totalExposure').innerText = totalCT.toFixed(2); document.getElementById('totalTime').innerText = totalT.toFixed(2); document.getElementById('finalTWA').innerText = twaValue.toFixed(3) + " units"; var statusText = ""; if (totalT 8) { statusText = "Note: The sampled time exceeds 8 hours. The result shows the 8-hour normalized exposure average."; } else { statusText = "Calculation based on a full 8-hour standard work shift."; } document.getElementById('twa-status').innerText = statusText; document.getElementById('twa-result-box').style.display = 'block'; }

Understanding Time Weighted Average (TWA)

In occupational health and safety, the Time Weighted Average (TWA) is a method used to calculate a worker's daily exposure to hazardous substances or noise. Since exposure levels rarely remain constant throughout a shift, the TWA provides a single average value that reflects the total accumulated dose normalized over an 8-hour period.

The TWA Formula

TWA = (C₁T₁ + C₂T₂ + … + CₙTₙ) / 8

• C: The concentration of the substance (in ppm or mg/m³) or the noise level (in dBA) during a specific interval.
• T: The duration of exposure to that specific concentration (in hours).
• 8: The standard denominator representing a typical 8-hour workday.

Why is TWA Important?

Regulatory bodies like OSHA (Occupational Safety and Health Administration) and NIOSH set Permissible Exposure Limits (PELs) based on TWA values. A worker might be exposed to a very high concentration of a chemical for 30 minutes, but if the rest of their shift has zero exposure, their 8-hour TWA might still fall within safe legal limits. Conversely, low-level exposure sustained over many hours can result in a TWA that exceeds safety thresholds.

Example TWA Calculation

Imagine a factory worker handling a solvent with the following exposure pattern:

Time Period	Concentration (ppm)	Duration (Hours)
Morning Setup	40 ppm	2 hours
Production Run	15 ppm	4 hours
Cleanup	5 ppm	2 hours

Step 1: Multiply Concentration × Time for each period.
(40 × 2) + (15 × 4) + (5 × 2) = 80 + 60 + 10 = 150

Step 2: Divide the total by 8 hours.
150 / 8 = 18.75 ppm

In this case, the 8-hour TWA is 18.75 ppm. If the OSHA PEL for this substance is 20 ppm, the worker is within the safe compliance range.

Frequently Asked Questions

What if my shift is 12 hours?

OSHA regulations generally still require normalization to an 8-hour exposure limit. However, some health organizations suggest adjusting the PEL downward for longer shifts to allow the body more time for recovery. For calculation, you still sum the total exposure and divide by 8 unless using a specific extended-shift model (like the Brief and Scala model).

What happens if the total duration is less than 8 hours?

The standard TWA formula assumes zero exposure for the unsampled remainder of the 8-hour shift. This is why the denominator remains 8 even if only 6 hours were measured.