Why the same shift gives two different numbers
Because the EU and OSHA run different arithmetic over the same measurements.
The EU uses a 3 dB exchange rate — the equal-energy LEX,8h, defined by reference to ISO 1999:1990 in Article 2 of the directive. OSHA 1910.95 uses 5 dB and a percentage-dose method.
One ordinary shift can come out 3.6 dB apart. That is the distance between a limit breach and a clean report, with nobody having made a mistake.
01Two exchange rates, two different assumptions
An exchange rate says how much extra level buys you half the time.
Under the EU's 3 dB rule, doubling the sound energy adds 3 dB. Halve the duration and the level may rise 3 dB for the same daily exposure. It follows directly from treating equal energy as equal risk, which is what ISO 1999 does.
OSHA's 5 dB rule is more forgiving of short, loud events: the level may rise 5 dB for each halving of time. The result is expressed as a percentage of an allowed daily dose rather than as a level, and only then converted back into a time-weighted average.
Neither is arithmetically wrong. They encode different assumptions about how intermittent noise damages hearing. They are not interchangeable, and a figure carried from one into the other is not the same quantity.
02One shift, worked both ways
Take a maintenance fitter: 1 hour at 100 dB(A) on a grinder, then 7 hours at 80 dB(A) back in the workshop. Nothing unusual about the day.
| EU · ISO 1999 | OSHA 1910.95 | |
|---|---|---|
| Exchange rate | 3 dB | 5 dB |
| Method | Equal-energy LEX,8h | Percentage dose, converted to TWA |
| Result | 91.3 dB(A) | 87.6 dB(A)71.9% dose |
| Against its own values | Above the 87 dB(A) exposure limit value | Below the 90 dB(A) PEL; above the 85 dB(A) action level |
Computed with the equal-energy formula and with OSHA’s C/T dose method respectively. The EU figure is reproducible in the free calculator on this site.
The same eight hours. A 3.6 dB gap, produced entirely by the exchange rate. In Europe this shift sits above the exposure limit value before hearing protection is even considered. In the United States it is not a PEL violation — though it does put the worker into a hearing conservation programme.
Which is why “we’re compliant” is an incomplete sentence. Compliant with which standard, computed which way?
03And the ceiling differs too — even inside the EU
The exchange rate is only half the story. The value your result is compared against is not the same in every member state, so two plants in one group, running identical readings, can file different conclusions.
Take a simpler case: a steady 86 dB(A) across a full eight-hour shift. Here both methods agree on the number, because there is nothing intermittent for the exchange rate to bite on. The verdicts still diverge.
| Jurisdiction | Its values | Verdict at a steady 86 dB(A) |
|---|---|---|
| EU directive2003/10/EC | 85 upper action · 87 limit | Above the upper action value; below the limit value |
| LithuaniaA1-103/V-265 | 85 viršutinė · 87 ribinė | Above the upper value; below the limit value |
| GermanyLärmVibrationsArbSchV | 85 oberer Auslösewert · 85 ceiling | Above the upper trigger; the ceiling test is made with protectors |
| PolandDz.U. 2018 poz. 1286 | 80 próg działania · 85 NDN | At or above the NDN — an environmental exceedance |
| OSHA1910.95 | 85 action level · 90 PEL | 57% dose. Above the action level; below the PEL |
A steady 86 dB(A) over eight hours, assessed without hearing protection in every column. Poland records an exceedance where Lithuania records only a trigger.
04What this means for a calculator
Halo HSE and the free calculator on this site compute the equal-energy LEX,8h. Selecting OSHA changes which limits your result is compared against. It does not change the arithmetic underneath. Neither product performs a 5 dB dose calculation.
That is stated on screen, next to the toggle, because the alternative is a number that looks like an OSHA result and is not one. If you need a true OSHA dose you need a tool that computes C/T fractions — we would rather tell you that than hand you the wrong quantity.
Work out a daily exposure in the browser, free and with nothing sent anywhere, or get Halo HSE for iOS.
05Sources
- Directive 2003/10/EC, Articles 2 and 3 Definition of LEX,8h by reference to ISO 1999:1990, and the exposure limit and action values. eur-lex.europa.eu
- OSHA 29 CFR 1910.95, Table G-16 and Appendix A The 90 dB(A) permissible exposure limit, the 85 dB(A) action level at 50% dose, and the C/T dose method. osha.gov
- Lärm- und Vibrations-Arbeitsschutzverordnung, § 6 and § 8 German trigger values and maximum permissible exposure value. gesetze-im-internet.de
- Darbuotojų apsaugos nuo triukšmo keliamos rizikos nuostatai, A1-103/V-265 Lithuanian lower, upper and limit exposure values. e-seimas.lrs.lt
- Dz.U. 2005 nr 157 poz. 1318 and Dz.U. 2018 poz. 1286 Polish action thresholds and NDN values. isap.sejm.gov.pl
ISO 1999 is named, not reproduced. This guide is written for competent persons and is not legal advice. If you believe something here is wrong, tell us — corrections are published with the date they were made.