**2.2 Sample collection**

The Aragonese population working in the metal industry during the study period 1991- 2000, was evaluated using the Industrial Companies Survey (Spanish acronym EIE) conducted by the Spanish National Statistics Institute (Spanish acronym INE), and an average population of 10,802 workers was obtained.

The data was provided by the Spanish National Institute of Safety and Hygiene at Work (Spanish acronym INSHT) and the Aragon Institute of Occupational Safety and Health (Spanish acronym ISSLA), from a list of companies in their files.

The initial sample size represented 10% of the workers, i.e. 1,080 individuals, using a systematic sampling of companies from said list.

From the initial selection, the following were eliminated: individuals not exposed to occupational noise; those who presented alterations in audiometric tests due to causes other than noise; individuals who, prior to their exposure to occupational noise (t=0), had been subjected to noise outside work over a long period of time; individuals exposed to solvents and degreasing agents and products that did not qualify for inclusion. The final study sample included 558 workers.

### **2.3 Description of variables**

A total of six variables were used, which can be divided into two groups. The first group, characterised by not having missing values consists of three variables, which define the cause-effect relationship: time of noise exposure, the atmosphere to which individuals were

Exploration Databases on Occupational Hearing Loss 195

dispersion graph illustrates the variation between them. Age was rejected as a suitable variable, since in addition to not defining the real duration of exposure effectively it had to then be transformed to achieve its lineal distribution, whereas this was not a problem when

3. "Degree of hearing alteration" (DALT). This was an ordinal qualitative variable with five modalities. Each modality was treated as binary. The variable identified the degree of hearing alteration, defined as the decline in the auditory threshold according to acoustic frequency, measured using an audiometer. The audiometry studied at times gave rise to two types of problems in relation to the interpretation of the results. These concerned manual corrections to the audiometric profile and the impossibility of observing the audiometric profile. Therefore the degree of hearing alteration was recorded according to a diagnostic code assigned by the doctor responsible for the check-up based on the Klockhoff classification (1973): H=healthy (losses 25 dB); IAT=initial acoustic trauma (losses of between 25 and 40 dB); AAT=advanced acoustic trauma (losses of between 40 and 50 dB); MH=mild hypoacusis (losses of between 50 and 55 dB); AH=advanced hypoacusis (losses > 55 dB). The losses indicated refer to the 4000 Hz frequency. There was also a loss of adjacent

4. "Smoking habit" (SH). A nominal binary variable. Recorded whether or not the subject

5. "Noise outside work" (NOW). A nominal binary variable. Recorded whether or not noisy

6. "Hearing protection" (HP). A nominal binary variable. Recorded whether or not hearing

Variables (Modifying) n % SMOKING HABIT (SH) 558 No 147 26.3 Yes 130 23.3 Missing values 281 50.4 NOISE OUTSIDE WORK (NOW) 558 No 192 34.4 Yes 35 6.3 Missing values 331 59.3 HEARING PROTECTION (HP) 558 No 103 18.5 Yes 95 17 Missing values 360 64.5

The events were defined based on the "degree of hearing alteration" variable, treating this as a nominal variable of binary response. Since in reality it is an ordinal variable with five modalities, it was necessary to transform the initial variable in such a way that the code (0)

the length of time exposed to noise was used as the variable.

frequencies as the degree of hearing alteration increases.

**2.3.2 Habits or modifying variables (Table 2)** 

activities were undertaken outside of work.

smoked.

protection was used.

Table 2. Habit variables

exposed and the degree of hearing alteration. The second group of variables, characterised by having missing values, refers to certain personal habits (smoking, exposure to nonoccupational noise and use of hearing protection). These can modify the response of the individual to the environmental factors to which they are exposed at work. These variables therefore have to be controlled to achieve the most accurate interpretation of the results.

#### **2.3.1 Exposure or cause-effect variables (Table 1)**

1. "Exposure atmosphere" (AEXP). This was a nominal qualitative variable with three categories. Each category was treated as binary. The variable noise was determined using an integrated sound level meter to classify the individuals in terms of their degree of exposure and its duration. Chemicals were assessed by the presence or absence of the corresponding particles of fluids or smoke in the atmosphere at work. The classification of noise intensity, moderate or high, was adopted for this work. Each one of the three atmospheres at work considered were classified: (a) MF= mainly noise of moderate intensity [85-90) dB(A) in the presence of metalworking fluids; (b) N=only noise, of moderate or high intensity ≥ 85dB(A); (c) WF= mainly noise of high intensity ≥ 90dB(A) in the presence of welding fumes.


Table 1. Exposure variables

2. "Exposure time to noise" (TEXP). This was a discrete quantitative variable expressed in years. It was an estimation of the time that the worker had been exposed to noise throughout his or her working life. It was established by consulting the individual directly. The possibility of using both the age of the workers and the length of time they were exposed to noise as the time variable was assessed. The projection of each together on a

exposed and the degree of hearing alteration. The second group of variables, characterised by having missing values, refers to certain personal habits (smoking, exposure to nonoccupational noise and use of hearing protection). These can modify the response of the individual to the environmental factors to which they are exposed at work. These variables therefore have to be controlled to achieve the most accurate interpretation of the results.

1. "Exposure atmosphere" (AEXP). This was a nominal qualitative variable with three categories. Each category was treated as binary. The variable noise was determined using an integrated sound level meter to classify the individuals in terms of their degree of exposure and its duration. Chemicals were assessed by the presence or absence of the corresponding particles of fluids or smoke in the atmosphere at work. The classification of noise intensity, moderate or high, was adopted for this work. Each one of the three atmospheres at work considered were classified: (a) MF= mainly noise of moderate intensity [85-90) dB(A) in the presence of metalworking fluids; (b) N=only noise, of moderate or high intensity ≥ 85dB(A);

Variables (Cause-Effect) n %

2. "Exposure time to noise" (TEXP). This was a discrete quantitative variable expressed in years. It was an estimation of the time that the worker had been exposed to noise throughout his or her working life. It was established by consulting the individual directly. The possibility of using both the age of the workers and the length of time they were exposed to noise as the time variable was assessed. The projection of each together on a

(c) WF= mainly noise of high intensity ≥ 90dB(A) in the presence of welding fumes.

EXPOSURE ATMOSPHERE (AEXP) 558 Noise Only, N 177 31.7 Noise+Metalworking Fluids, MF 146 26.2 Noise+Welding Fumes, WF 235 42.1 EXPOSURE TIME (TEXP) 558 0-5 57 10.2 5-10 41 7.3 10-15 36 6.5 15-20 42 7.5 20-25 85 15.2 25-30 116 20.8 30-35 106 19 35-40 50 9 40-45 22 3.9 45-50 3 0.5 DEGREE OF ALTERATION (DALT) 558 H 158 28.3 IAT 196 35.1 AAT 105 18.8 MH 70 12.5 AH 29 5.2

**2.3.1 Exposure or cause-effect variables (Table 1)** 

Table 1. Exposure variables

dispersion graph illustrates the variation between them. Age was rejected as a suitable variable, since in addition to not defining the real duration of exposure effectively it had to then be transformed to achieve its lineal distribution, whereas this was not a problem when the length of time exposed to noise was used as the variable.

3. "Degree of hearing alteration" (DALT). This was an ordinal qualitative variable with five modalities. Each modality was treated as binary. The variable identified the degree of hearing alteration, defined as the decline in the auditory threshold according to acoustic frequency, measured using an audiometer. The audiometry studied at times gave rise to two types of problems in relation to the interpretation of the results. These concerned manual corrections to the audiometric profile and the impossibility of observing the audiometric profile. Therefore the degree of hearing alteration was recorded according to a diagnostic code assigned by the doctor responsible for the check-up based on the Klockhoff classification (1973): H=healthy (losses 25 dB); IAT=initial acoustic trauma (losses of between 25 and 40 dB); AAT=advanced acoustic trauma (losses of between 40 and 50 dB); MH=mild hypoacusis (losses of between 50 and 55 dB); AH=advanced hypoacusis (losses > 55 dB). The losses indicated refer to the 4000 Hz frequency. There was also a loss of adjacent frequencies as the degree of hearing alteration increases.
