Occupational exposure to BTEX and styrene in West Asian countries: a brief review of current state and limits

To get as complete coverage of current OELs across the countries in West Asia, we compiled available information from the GESTIS database and online searches for OEL lists from West Asian countries. The European Chemicals Agency (ECHA) webpage was searched for recent EU level recommendations and derived no-effect levels (DNELs) based on the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) regulation (20). Scientific documentation about OELs from the US ACGIH, German maximum workplace concentrations (MAK), Scientific Committee on Occupational Exposure Limits (SCOEL) and ECHA Committee for Risk Assessment (RAC) were compiled and reviewed for information on critical effects, skin notation, carcinogen classifications, and biological monitoring guidance values.

We systematically searched international datasets including PubMed, Scopus, Cochrane Library, CINAHL, ISI Web of Science, ScienceDirect, PROSPERO, and EMBASE to identify articles in English related to occupational exposure to BTEX and styrene in West Asian countries and published between 1980 and 2021. For this purpose, we used several combinations of the key words (benzene, toluene, ethylbenzene, xylene, styrene, occupational exposure, industrial exposure, threshold limit value, occupational exposure limit, recommended exposure level, permissible exposure limit, West Asia, and Middle East) and search criteria, including language and publication year (Table 1).

First we identified 4199 references that matched our search key terms. Figure 1 shows how we proceeded until we got the final number of 49 full-text articles that met the inclusion criteria: they all had to be original articles in English reporting airborne workplace concentrations of the studied chemicals from 1980 to 2021. We excluded articles not containing original data, such as review articles, case series, and case reports. Each included article was individually assessed for completeness of reporting by two independent reviewers according to the Strengthening the Reporting of Observational Studies in Epidemiology (STROBE) checklist (21).

Figure 1

Both static and personal monitoring studies were included. Quality of sampling was considered sufficient if sampling pump flow rate was between 0.1 and 0.3 L/min, and in case of static monitoring if samplers were placed at the height of 1.5 to 2 m.

After the screening, the data were extracted and cross-checked. Any inconsistencies were resolved by consultation with a third reviewer. The following information was extracted and tabulated: bibliographic information (reference ID, authors, year published, publication type), country where study was performed, industry sector, type of sampling (static or personal monitoring), type of sampler (active/pump or passive sampling), number of exposed workers and non-exposed workers (not applicable to static monitoring), number of area samples (not applicable to personal monitoring), number of individual samples (may be more than one per worker or area), duration of sampling, gender distribution (male/ female; not applicable to static monitoring), mean and standard deviation of age (of exposed and unexposed groups, not applicable to static monitoring), means and standard deviations of BTEX and styrene concentrations across all sections of investigated workplace, and, if applicable, range of means for subsections of investigated workplace and whether the reported concentrations were recalculated to represent the eight-hour (workday) exposure scenario (where the workday was shorter than 8 h).

All reported airborne concentrations were converted to ppm, using the following equation:

This assumes 25 °C and air pressure of 101.325 kPa (760 Torr).

Descriptive results of variables are given as means and standard deviations. For all analyses we used the Statistical Package for Social Sciences (SPSS), version 22 (IBM, Armonk, NY, USA).

Evaluating compliance with an OEL requires assessment of exposure variability for an exposed group. There is also a normative aspect, that is, how certain we wish to be that exposure is unlikely to be exceeded for a small part of the exposed group and how small that group should be. Industrial hygiene guidance documents refer to 70 % certainty that no more than 5 % of similarly exposed workers would experience exposures exceeding the OEL (22, 23). We, however, approached OEL compliance in a simpler fashion, by comparing reported average exposure to the applicable OEL, as the reviewed papers do not report their results in sufficient detail. As airborne exposures are generally log-normally distributed, the arithmetic mean will be a slightly more conservative than the true central estimates of the median or geometric mean. Where reported, we also compared the full range (min–max) of reported exposure to applicable OELs.

Benzene is a well-known carcinogen, although conclusions as to whether its mechanism of action supports a threshold or not differs between OEL expert groups. However, more recent assessments tend to conclude that benzene can be viewed as a threshold carcinogen, and consequently that sufficiently low exposures would protect against cancer risk. Most eight-hour time-weighted averages (TWA) for OELs are 1.6 mg/m³ (0.5 ppm) while the most recent recommendation of ECHA RAC is that of 0.16 mg/ m³ (0.05 ppm) (Table 2).

Carcinogenicity is generally not considered critical for the other substances. However, the International Agency for Research on Cancer (IARC) has classified ethylbenzene as possibly carcinogenic to humans (group 2B) and styrene as probably carcinogenic to humans (group 2A). All of the reviewed substances are potentially neurotoxic, and most of the OELs for toluene, xylene, and styrene take into account some form of neurotoxicity and irritation as critical effects. Assignment of skin notation varies: ACGIH has a skin notation only for benzene, whereas the German MAK commission and SCOEL have assigned skin notations to all BTEX substances but not to styrene. All the substances have several biological guidance values, mostly in urine.

Our review of the 49 full-text articles shows that occupational exposure levels to BTEX and styrene in West Asian countries have not been measured and reported in a structured or uniform manner, and that most reports refer to Iran. Furthermore, studies investigating occupational exposure to styrene are few and far between. The summary of the extracted data is available in Table 3 (24–65). Most studies address exposure in oil-related industries (such as petrochemical, oil refineries, petrol and compressed natural gas stations, and petroleum depots), while the rest looks into exposure in the shoe factories, plastic industries, pesticides production factories, printing, electronics, or steel industries or among beauty salon workers, drivers, and traffic policemen. Average occupational exposure to benzene in oil-related industries is higher than the OELs recommended in Table 2. As shown in Table 4, the mean air concentrations of toluene, ethylbenzene, xylene isomers, and styrene reported by most studies are lower than the recommended OELs for the same country (if any, see Table 2).

Table 4 shows the analysis of reported occupational exposure to BTEX and styrene narrowed down to oil-related and solvent-related industries (such as shoe factories, printing, electronics, automobile industry, pesticide production factory, tyre factories, steel industries, chemical industry, plastic industry, and beauty salons).