NOISE 775
exposure time is halved with each 5 dBA sound level increase.
The threshold level, the sound level below which no contribu-
tion is made to daily noise dose, is 80 dBA (threshold level is
not to be confused with hearing threshold). When noise expo-
sure exceeds the action level (85 dBA) a hearing conservation
program is to be implemented. A hearing conservation program
should include noise exposure monitoring audiometric testing,
employee training, hearing protection and record-keeping.
According to OSHA standards continuous noise expo-
sure (measurable on the slow-response scale of a sound level
meter) is not to exceed 1151 dBA. For sound levels L where
80 L 115 dBA allowable exposure time is given byT
82 ⁄⎡ L 82 ^5
⎣ ⎤⎦ ⁄⎡⎣ ⎤⎦( CL) ER⁄ (L90)⁄
(9.1)where T allowable exposure time (hours/day). The result
is shown in Table 6.
Noise dose. When sound levels vary during the day, noise
dose is used as an exposure criterion. Noise dose is given byDCT
iN
%
(^100) ∑ ii⁄
1
where C actual exposure of an individual at a given sound
level (hr), T allowable exposure time at that level and N
the number of different exposure levels during one day. Noise
dose D % should not exceed 100%. As an alternative to moni-
toring and calculations, workers may wear dosimeters which
automatically measure and calculate daily dose.
An exchange rate of 3 dB is used in occupational noise
exposure criteria by some European countries. This exchange
rate is equivalent to basing noise exposure on L eq.
Environmental Protection Agency (EPA) identified
levels. Using a 4 kHz threshold shift criterion, protective
noise levels are substantially lower than the OSHA criteria.
EPA (1974, 1978) in its “Levels” document identified the
equivalent sound level of intermittent noise:
Leq24 70 dBA
as the “(at ear) exposure level that would produce no more
than 5 dB noise-induced hearing damage over a 40 year
period”. This value is based on a predicted hearing loss
smaller than 5 dB at 4 kHz for 96% of the people exposed
to 73 dBA noise for 8 hr/da 250 da/yr 40 yr. With the
following corrections, the 73 dBA level is adjusted to L eq24
70 dBA the protective noise level:
1.6 dBA to account for 365 da/yr exposure,
4.8 dBA to correct for 24 hr/day averaging,
5 dBA assuming intermittent exposure and
1.6 dBA for a margin of safety.
NON-AUDITORY EFFECTS OF NOISE
The relationship between long-term exposure to industrial
noise and the probability of noise-induced hearing loss is
TABLE 5
Background noise correction
DIF COR DIF COR
0.2 13.5 — —
0.4 10.6 — —
0.6 8.9 — —
0.8 7.7 6.5 1.1
1.0 6.9 7.0 1.0
1.2 6.2 7.5 0.9
1.4 5.6 8.0 0.7
1.6 5.1 8.5 0.7
1.8 4.7 9.0 0.6
2.0 4.3 9.5 0.5
2.2 4.0 10.0 0.5
2.4 3.7 10.5 0.4
2.6 3.5 11.0 0.4
2.8 3.2 11.5 0.3
3.0 3.0 12.0 0.3
3.2 2.8 12.5 0.3
3.4 2.7 13.0 0.2
3.6 2.5 13.5 0.2
3.8 2.3 14.0 0.2
4.0 2.2 14.5 0.2
4.2 2.1 15.0 0.1
4.4 2.0 15.5 0.1
4.6 1.8 16.0 0.1
4.8 1.7 16.5 0.1
5.0 1.7 17.0 0.1
5.2 1.6 17.5 0.1
5.4 1.5 18.0 0.1
5.6 1.4 18.5 0.1
5.8 1.3 19.0 0.1
6.0 1.3 19.5 0.0
DIF total noise level–background
noise level. Sound level due to
source total noise level COR.
TABLE 6
Allowable exposure times
Time T
hr/da
Sound level L
dBA
32 80
16 85
890
495
2 100
1 105
1/2 110
1/4 or less 115
The 32 hr exposure time is used
in evaluating noise dose when
sound levels vary.
C014_003_r03.indd 775C014_003_r03.indd 775 11/18/2005 10:46:10 AM11/18/2005 10:46:10 AM