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A single substance's daily average, or several substances combined.
The 8-hour TWA is how every long-term limit is expressed. The mixture index is for substances that act on the same organ in the same way — several solvents, for instance — where each can be under its own limit while together they are not.
The unit the sampling results and the limit are both in.
Concentrations are the same in every market, but the result and the limit must share one unit; this sets the label, not a conversion.
The average concentration in the breathing zone over this period, in the unit chosen above.
From personal sampling in the breathing zone over the period — a task, or a sampling run — analysed by a recognised method. Periods not sampled but known to be unexposed can be left out: the 8-hour average counts them as zero.
How long the concentration applied — minutes divided by 60.
The duration of the sampling period or task the concentration describes. A row with no time against it is ignored.
The average concentration in the breathing zone over this period, in the unit chosen above.
From personal sampling in the breathing zone over the period — a task, or a sampling run — analysed by a recognised method. Periods not sampled but known to be unexposed can be left out: the 8-hour average counts them as zero.
How long the concentration applied — minutes divided by 60.
The duration of the sampling period or task the concentration describes. A row with no time against it is ignored.
The average concentration in the breathing zone over this period, in the unit chosen above.
From personal sampling in the breathing zone over the period — a task, or a sampling run — analysed by a recognised method. Periods not sampled but known to be unexposed can be left out: the 8-hour average counts them as zero.
How long the concentration applied — minutes divided by 60.
The duration of the sampling period or task the concentration describes. A row with no time against it is ignored.
The average concentration in the breathing zone over this period, in the unit chosen above.
From personal sampling in the breathing zone over the period — a task, or a sampling run — analysed by a recognised method. Periods not sampled but known to be unexposed can be left out: the 8-hour average counts them as zero.
How long the concentration applied — minutes divided by 60.
The duration of the sampling period or task the concentration describes. A row with no time against it is ignored.
The substance's 8-hour limit in the same unit — 0 shows the TWA without a comparison.
The workplace exposure limit from EH40 in Great Britain, or the OSHA permissible exposure limit in the US. They differ: respirable crystalline silica is 0.1 mg/m³ in EH40 and 0.05 mg/m³ under OSHA's construction silica standard. Use the one that applies where the work is done, and the current edition.
Exposure result
0.09375 mg/m³
The periods entered average to 0.0938 mg/m³ over the 8-hour reference period, counting the rest of the day as unexposed.
- Hours with an exposure entered
- 5 hours
- Period 1's share of the day's dose
- 80 %
- Period 2's share of the day's dose
- 20 %
They open the calculator with your figures already in it
Workplace Exposure 8-Hour TWA Calculator (Dust, Fume, Vapour): 0.0938 mg/m³ — shown in imperial, US market. The link sets both, so the result they see is the one on your screen.
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How this was calculated
Formula source(s)
- HSE EH40/2005 (Workplace exposure limits): the 8-hour TWA = (C1T1 + C2T2 + … + CnTn) ÷ 8, where C is the concentration over each period of T hours and time not exposed counts as zero
- 29 CFR 1910.1000(d)(1)(i): E = (CaTa + CbTb + … + CnTn) ÷ 8 for the 8-hour time-weighted average; 29 CFR 1910.1000(d)(2)(i): for a mixture, Em = C1/L1 + C2/L2 + … + Cn/Ln, which is not to exceed unity
- EH40/2005 and 29 CFR 1926.1153: respirable crystalline silica has a GB workplace exposure limit of 0.1 mg/m³ and a US construction permissible exposure limit of 50 µg/m³ (0.05 mg/m³), each as an 8-hour TWA
Inputs used
- What to Work Out
- One substance over the day — the 8-hour TWA
- Concentration Unit
- mg/m³ — dusts, fumes and most limits
- Period 1 Concentration
- 0.3
- Period 1 Duration (hours)
- 2
- Period 2 Concentration
- 0.05
- Period 2 Duration (hours)
- 3
- Period 3 Concentration
- 0
- Period 3 Duration (hours)
- 0
- Period 4 Concentration
- 0
- Period 4 Duration (hours)
- 0
- 8-Hour Limit to Compare Against (0 for none)
- 0
- Substance 1 8-Hour TWA
- 50
- Substance 1 8-Hour Limit
- 100
- Substance 2 8-Hour TWA
- 20
- Substance 2 8-Hour Limit
- 50
- Substance 3 8-Hour TWA
- 0
- Substance 3 8-Hour Limit
- 0
- Substance 4 8-Hour TWA
- 0
- Substance 4 8-Hour Limit
- 0
Intermediate steps
- Hours with an exposure entered
- 5 hours
- Period 1's share of the day's dose
- 80 %
- Period 2's share of the day's dose
- 20 %
Confidence note: The periods entered average to 0.0938 mg/m³ over the 8-hour reference period, counting the rest of the day as unexposed.
What this calculation does not cover
- It averages results you already have; the sampling behind them has to follow a recognised method, in the breathing zone, over periods that represent the work.
- Respiratory protection is not deducted. Limits apply to the air the worker would breathe without it, and what a respirator achieves in practice depends on fit testing and wear.
- Short-term limits — 15-minute STELs and ceiling values — are separate tests on the peaks and are not assessed here; a day comfortably under its 8-hour limit can still breach a short-term one.
- The mixture index applies only to substances with the same kind of effect on the same organ. Substances acting independently are each judged against their own limit, and a mixture index across them means nothing.
Computed in your browser — nothing you enter is uploaded. Presented in US customary units and US trade terminology. Where a formula follows a published standard, that standard and its edition are cited beside it on this page; where none governs, the page says so. Local amendments override model codes — verify against the code in force where you build.
Sources checked 2026-09-21 · v1.0.0
Regulatory standards & verification citations3
- HSE EH40/2005 (Workplace exposure limits): the 8-hour TWA = (C1T1 + C2T2 + … + CnTn) ÷ 8, where C is the concentration over each period of T hours and time not exposed counts as zero
- 29 CFR 1910.1000(d)(1)(i): E = (CaTa + CbTb + … + CnTn) ÷ 8 for the 8-hour time-weighted average; 29 CFR 1910.1000(d)(2)(i): for a mixture, Em = C1/L1 + C2/L2 + … + Cn/Ln, which is not to exceed unity
- EH40/2005 and 29 CFR 1926.1153: respirable crystalline silica has a GB workplace exposure limit of 0.1 mg/m³ and a US construction permissible exposure limit of 50 µg/m³ (0.05 mg/m³), each as an 8-hour TWA
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