**1. Introduction**

190 Hearing Loss

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> Studies on occupational hearing loss have focused on noise as the primary cause. While the effect of this physical agent on hearing has been demonstrated, an analysis closer to the site of exposure confirms that the presence of other contaminants, such as chemicals, can interact with noise. This association may influence a temporal variability in the manifestation of an occupational hearing pathology.

> In this respect, the term "working conditions" is too ambiguous (i.e., noise in the metal industry) as, in apparently similar conditions, several exposure environments can be identified: machining (noise+fluids, e.g. lathing), manufacture of structures (noise+fumes, e.g. welding) and surface protection (noise+solvents, e.g. painting), among others.

> The European Agency for Safety and Health at Work recognises that noise-induced hearing loss is the most common occupational disorder in Europe. It advises that, in order to achieve greater efficiency in its prevention, more attention must be paid to the combined risk factors (multiple exposures) in workers exposed to high noise levels and chemical compounds associated with their work.

> Similarly, recent studies conducted in the US (Agrawall et al., 2009) and New Zealand (Thorne et al., 2008) recognise noise-induced hearing loss as one of the most widespread occupational illnesses in these countries. Conclude that traditional noise monitoring and control methods have not achieved the expected results, identifying increasing prevalence in the general working population, and particularly in young people.

> This study aims to test the hypothesis of interaction between various physical and chemical pollutants and their influence on hearing. It obtains a complete temporal exposure model, based on survival analysis, which covers the entire working life of an individual between t=0 (start time) and t=50 years (maximum period). The study of multiple exposures using a qualitative variable allows the prevention cost associated with hygiene risk assessment (see 2.3.1. point 1) to be sufficiently reduced. This is also the methodology used in the study of other environment related illnesses caused by prolonged exposure to different agents.

Exploration Databases on Occupational Hearing Loss 193

In view of the above, the aim of this study is to analyse the influence of the combination of different chemical agents and noise on occupational hearing loss within the metal industry,

A descriptive epidemiological study was conducted, using two types of sources: one based on the records of each individual, occupational medical examinations (OME), with a specific noise protocol (SNP), carried out on various dates during the inclusion period, providing

The second type involved on-site testing of a selection of job positions, in order to ensure the type and homogeneity of the environmental exposure conditions of the individuals in the companies included in the sample during the period of study, and environmental record of

The study design presented is conceptually interpreted as longitudinal, as defined by Rothman (1986), the existence of a time interval between exposure and the onset of illness. With two observation points, at t=0 (estimated starting point for the specified sources, after having first carried out a strict process of selection of individuals to be included in the

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

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

The initial sample size represented 10% of the workers, i.e. 1,080 individuals, using a

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

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

to be aware of the interrelationships between such factors for preventive purposes.

their audiometric data, duration of exposure to noise, and personal habits.

study) and at t=n (period in which the first audiometric test was performed).

average population of 10,802 workers was obtained.

systematic sampling of companies from said list.

sample included 558 workers.

**2.3 Description of variables** 

(Spanish acronym ISSLA), from a list of companies in their files.

**2. Material** 

**2.1 Study design** 

exposure (ERE).

**2.2 Sample collection** 

The analysis was carried out using as sample data taken from a pre-existing database on occupational health. The aim was to assess the viability of using these historical databases and the quality of the information obtained from them with regard to the interaction between noise and chemicals and the effect of this interaction on hearing.

The characteristics of the archive information determined the design of the study, the definition of the variables and the method of data analysis used. For instance, the instruments used to measure these variables in some cases may have changed over the prolonged time of this study and it is therefore difficult to maintain consistency. These instruments include: audiometers for identifying the decline in the auditory threshold; integrated sound level meters and dosimeters for the measurement of environmental noise; vacuum pumps for taking air samples, and instruments for chemical analysis used for collecting and quantifying environmental chemical contaminants. Consequently quantitative recording was avoided, defining measurements qualitatively (as binary variables) instead. This provided greater flexibility when evaluating variables, eliminating possible discrepancies associated with potential changes in technology and measurement criteria.

Using a minimum amount of information, one discrete quantitative variable (length of time exposed to noise) and the remaining qualitative variables, it was possible to estimate the influence of a particular working environment on hearing in combination with certain personal habits. The results obtained are of descriptive and explanatory interest, providing information on the interactions between the stated variables and their effects on the individual.

The analysis of the data was fast and economical, whereas obtaining pure samples of data would be less so. Furthermore, and as a corollary, average or high frequencies is required in order to give consistency to the analysis. In addition, if a classification is used to record a variable, it has to be entirely discrete. Failure to fulfil these two criteria (frequency and being discrete) can make analysis using the proposed methodology ineffective, as speculation about the data could lead to an unreliable interpretation of that data.

The results obtained show that workers exposed to noise where metalworking fluids are present show a greater delay in hearing alteration than workers exposed only to noise. By contrast, workers exposed to noise where welding fumes are present exhibited an increase in hearing alteration compared to those exposed only to noise. This thereby demonstrates the antagonistic effect of metalworking fluids with noise and the synergic effect of welding fumes.

As a preventative application, there exists a need for combined respiratory and auditory protection in processes that produce welding fumes, and the former should be effective against certain gases and metal components (use of integrated personal protection equipment). Fabric masks (a highly-used protection) do not meet this requirement, and nor do extraction systems. Environments with noise and metalworking fluids have the advantage in that the aforementioned masks can be used as respiratory protection combined with auditory protection.

Based on recognised research for the study of this problem (Gobba, 2003), the study of pathogenic mechanisms, and evaluation of new multiple-exposure thresholds. This paper focuses on the second of these aspects, the purpose being to obtain patterns that allow for the comparison of various populations of workers in multiple-exposure conditions similar to those defined by such patterns.

In view of the above, the aim of this study is to analyse the influence of the combination of different chemical agents and noise on occupational hearing loss within the metal industry, to be aware of the interrelationships between such factors for preventive purposes.
