This short article covers a fundamental, but not straightforward, issue in noise induced hearing loss cases – calculating the Noise Immission Level. It is often the make, or break, of a case.
What is the Noise Immission Level
The Noise Immission Level (NIL) is a calculation as to an individual’s lifetime exposure to noise. The background to different definitions of ‘Noise’ are covered in more detail in my earlier article: What is ‘Noise’ in NIHL cases?
However, the NIL is relevant because, depending on audiometric configuration (R3 of the Coles Guidelines ), a claimant will need to show a NIL of 100 dB (A) NIL or, less frequently, 90 dB (A) NIL.
The formula is included in the ‘NPL Tables’ of 1977, as referenced in the Coles guidelines. It is:
NIL = (Average daily noise level) + (10 * log10 T)
Where the average daily noise level is measured in “dB (A) lep,d” and “T” is time (in years).
(For those with iPhones, turning the phone to landscape whilst using the calculator also shows a scientific calculator with a log10 button…).
So for an individual who was exposed to 91.5 dB (A) lep,d for 11 years, this will result in a NIL of:
= 91.5 + (10 * log10 11)
Log10 of 11 is 1.041
So the NIL is:
= (91.5 + 10.41)
= 101.91 dB (A) NIL
85 dB (A) lep,d for 5 years – 92.0 dB (A) NIL
85 dB (A) lep,d for 20 years – 98.0 dB (A) NIL
90 dB (A) lep,d for 1 year – 90 dB (A) NIL
90 dB (A) lep,d for 10 years – 100.0 dB (A) NIL
95 dB (A) lep,d for 3 years – 99.8 dB (A) NIL
Clearly for a NIL figure to be ‘in evidence’ it needs to be contained in an expert report by an engineer who first needs to have calculated the daily noise level accurately. The time that the claimant was exposed to the noise also needs to be accurately described.
However, a working knowledge of the formula can be useful for practitioners when considering ‘obvious’ passes or fails. It may be helpful when noise surveys show approximate noise levels, or the machinery used is known to expose users to known levels of noise.
So, if someone has been exposed to 90 dB (A) lep,d or more for, say, 20 years, then it would seem obvious that this would result in a NIL well in excess of 100 dB (A) NIL.
Likewise, if someone were exposed to, say, 85 dB (A) lep,d for 5 years – then this is going to be some way off a NIL of 100 dB (A) NIL.
In practice, there may well be multiple noisy employers, all with different levels of noise exposure. Calculating NILs in such a situation is not for the feint hearted. It really is the realm of engineers alone.
However, for ‘obvious’ cases again, it may be possible to calculate whether a claimant clearly has enough/ not enough exposure to meet the 100 dB (A) NIL or 90 dB (A) NIL required in any particular case.
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