TWA Calculator: 8-Hour Time-Weighted Average Exposure

The 8-hour time-weighted average concentration of an airborne contaminant, compared with the exposure standard.

Calculated from standard textbook formulas, with every step shown. The formulas are fixed, so the working does not need checking: what matters is the numbers you type in. Use these calculators only if you are competent to choose the inputs (loads, material strength, dimensions, supports) and check your units and assumptions. For learning and checking your own working: design and sign-off are for a qualified engineer. Drafting Central does not store the numbers you type.

How this calculator works

A worker is often exposed to different concentrations of a fume, dust or vapour through a shift. The time-weighted average spreads the exposure over 8 hours: multiply each concentration by the time at it, add them up, and divide by 8. Time with no exposure counts as zero.

The result is compared with the 8-hour exposure standard for the substance. The ratio shows how close the exposure is to the standard. Concentration and standard must be in the same unit, such as mg/m3 or ppm.

The standard is an input because it comes from the current workplace exposure standards, which can change. Some substances also have short-term or peak limits, or can be absorbed through the skin, and these are separate. The concentrations must come from proper air monitoring.

Formulas used

  • TWA = \frac{\sum C_i\,t_i}{8}
  • \text{share} = TWA / \text{standard}

Worked example

With the default numbers in the calculator above:

  1. Level 1: concentration x time: 180.00
  2. Level 2: concentration x time: 80.00
  3. Level 3: concentration x time: 0.00
  4. Eight-hour time-weighted average: 32.50
  5. Share of the exposure standard: 65.00 %

Change the numbers in the calculator to see the full working, every step, for your own case.

A second worked example

A different case, with these numbers: Concentration 1 (same unit as the standard, for example mg/m3 or ppm) 100; Time at concentration 1 in a shift 8 h; Time at concentration 2 in a shift 0 h; Time at concentration 3 in a shift 0 h.

  1. Level 1: concentration x time: 800.00
  2. Eight-hour time-weighted average: 100.00
  3. Share of the exposure standard: 200.00 %

Common questions

What is a TWA?

The average concentration over an 8-hour day, taking the time at each concentration into account.

What if my shift is longer than 8 hours?

The standard is for an 8-hour day, and a longer shift needs it adjusted by a recognised method. An occupational hygienist can advise.

Does a TWA below the standard mean it is safe?

It means the average is below the standard on the numbers entered. Short-term limits, skin absorption and mixtures still need to be considered, and the duty to minimise exposure remains.

Related calculators

Got a question about this? Ask an engineering question and get a plain-language answer with references.

Further reading: Safe Work Australia workplace exposure standards for airborne contaminants; Managing risks of hazardous chemicals in the workplace Code of Practice (Queensland); AS 2986 and AS 3640 (air sampling, licensed).

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