Toluene and xylene (o-, m- and p-isomers) are widespread in the environment because they have been used in many commercial products. Vehicle exhaust is considered to be the main source but non-professional use of paints, glues, adhesives, varnishes, lacquers, shoe polishes and cigarette smoke contribute significantly, especially to levels in the internal environment and to personal exposure [^1-3].

At high concentrations, toluene and xylene have serious adverse effects on human health. Toluene vapor is heavier than air and may travel along the ground, but harmful air contamination levels can be reached rather quickly by evaporation of toluene at 20?C. Toluene has a lower clearance rate in winter than in summer. The acute and chronic effects of toluene, and to a lesser degree of xylene, on the central nervous system are of most concern. In addition, there is concern about potential effects on reproduction and hormone balance in women, and hormone imbalances in exposed males have also been reported [^4-6].

In occupational settings, official agencies (OSHA, ACGIH, HSE, DFG) have established an 8-hour time-weighted average permissible exposure limit (TWA PEL) of 100 ppm (375 mg/m³ and 435 mg/m³ for toluene and xylene isomers, respectively). Levels only slightly above the 8-hour TWA may cause lack of coordination and amnesia. Even levels lower than 50 ppm cause drowsiness, moderate fatigue and headaches [⁷]. The World Health Organization has recently recommended a guideline for ambient toluene exposure to protect against developmental neurotoxicity: 260 ?g/m³ (68 ppb) as a weekly average concentration.

Athens, like most large cities all over the world, has air pollution problems. These were made worse by poor town layout, location (surrounded by mountains), intense sunlight and low wind speed. Emission of toluene has been calculated as 20 million tons per year, one of the largest among European towns [⁸]. In Athens, fixed point monitoring stations have very recently started to measure hydrocarbon concentrations, but very little comparative research has been conducted to give consistent estimates of exposures of individuals in the general population or of indoor pollution [^9-13]. The main goal of the present study was to estimate personal exposures to toluene and xylene by monitoring fifty volunteers, their homes and various urban sites over one year. Another goal was to examine the relationship between toluene and xylene pollution in indoor, urban and personal air. Benzene measurements included in the study have already been published elsewhere [¹⁴].

The study population had an average age of 36.3 years and consisted predominantly of men (66%). Each sampling campaign lasted 108 hours. In each measurement period, subjects spent an average of 26.3 hours outdoors (sd 13.8). Time spent outdoors differed markedly between occupational groups. Teachers and students spent, as an annual average, less than 15 hours per sampling period outdoors, 50% during their free time, while bus drivers and traffic policemen exceeded 40 hours outdoors, almost 80% during work activities. In Table 1, the job and house characteristics of volunteers are presented along with the corresponding measurements of toluene and xylene (see also additional file 1, Table 1, for further residential characteristics). An arterial road with traffic density of more than 5000 vehicles per day witch judged independently by two professional drivers as busy road.

Urban levels, indoor residential concentrations and personal exposures to toluene and xylene during the six sampling periods are presented in Tables 2 and 3. Table 4 shows average wind speed, humidity, amount of sunlight and temperature for all six measurements. As shown in these tables, personal exposures varied between 53 ? 80 and 56 ? 85 ?g/m³ for toluene and xylene, respectively. The higher values were observed mainly during the first two periods i.e. Autumn and Winter, when the wind speed did not exceed 0.5 m/s. Indoor residential concentrations varied between 30 and 51 ?g/m³ and 27 and 48 ?g/m³ for toluene and xylene, respectively. Urban levels ranged from 47 to 84 and 53 to 88 ?g/m³ for toluene and xylene, respectively. Personal exposures followed similar trends to urban and indoor pollution throughout the study period (Figures 1 and 2).

Correlations between personal exposures and indoor concentrations were weaker for xylene than for toluene, and, as expected, for other seasons than for winter. The correlation coefficient (r) between personal exposure and residential level varied between 0.33 and 0.82 (all campaigns 0.57) for toluene and between 0.20 and 0.65 (all campaigns 0.39) for xylene (see additional file 1, Table 2).

Personal exposures to and indoor concentrations of toluene and xylene are presented in Figures 3 and 4 according to occupational group. Teachers and students differed significantly from other groups in respect of personal exposure to both toluene and xylene (p < 0.01).

In this study, non-smoking citizens of Athens were selected among people who, owing to occupational duties, spent a lot of time in the streets, and people who spent more time indoors in schools or offices. Those who spent more working or leisure time in the heavily polluted city center were expected to be highly exposed, given the pattern of pollution of Athens (i.e. pollution comes from outdoors). Although inferences about a population are problematic when volunteers are used because of the potential for selective participation, the use of volunteers is suitable for investigating the relationships among predictors of personal exposure [²⁰].

Urban levels of toluene and xylene were generally high. The average annual indoor residential concentrations were found to be 39 ?g/m³ and 36 ?g/m³ for toluene and xylene, respectively. These levels are in the same order as those found in other studies in Athens and cities with similar characteristics, and lower than some other Mediterranean cities [^9-13,^21-28]. The average annual personal exposures were found to be 63 ?g/m³ and 67 ?g/m³ for toluene and xylene, respectively. Urban toluene pollution very rarely (<2% of all measurements) exceeded the limit of 260 ?g/m³ proposed by WHO as a weekly average. In this particular setting, the toluene and xylene measurements probably represented the expected levels of exposure of the vast majority of non-smoking citizens of Athens who were not occupationally exposed.

Urban and indoor air pollution and personal exposure exhibited the same pattern of variation among the measurement periods. This variation among monitoring campaigns might be explained by differences in climate parameters, namely wind speed, humidity and amount of sunlight. It was also anticipated that changes in transport and modernization of the bus fleet during the study period would to some extent have influenced atmospheric pollution levels, especially in the city centre. The most important change was a project for replacing the old buses entirely with new ones equipped with anti-pollution devices. More than 100 buses were replaced during the study period; in addition, increased numbers of dedicated bus lanes improved running conditions.

Indoor concentrations of toluene and xylene are often significantly higher than external air levels [²⁹,³⁰]. Lebret found an internal/external ratio of 8 in Holland, while Gilli found a ratio of 3 in Torino, Italy [³¹,³²]. In contrast, our results showed an indoor/outdoor toluene air pollution ratio of 0.64 for houses within the environmental sampling areas, although the samples were not obtained directly outside the homes. The ratio for xylene concentrations was 0.54. Probably poor emission materials and characteristics related to the Athens climate, such as frequent ventilation and use of non-absorbent materials for wall and floor coverings, account for this result. The frequent ventilation of houses in Athens lowered indoor pollution because such ventilation usually takes place when urban pollution is lower (not rush hour) and there are putative indoor emission sources, so the infiltration of outdoor air is beneficial. This argument is supported by the finding that differences between outdoor and indoor levels were lower during the spring and summer periods (continuous infiltration of outdoor air). Compared to others studies, we found weak but not significant effects on indoor air pollution from environmental smoke, heating mode, type and floor of house, and recent painting [²,³,^29-34].

Indoor air contributes significantly to human exposure. In poorly-ventilated buildings, indoor emission sources have a more significant influence on hydrocarbon concentrations than infiltration of outdoor air [[²⁶,³⁵] and [¹]]. In our study, correlations between personal exposure and indoor concentrations were higher for toluene than for xylene and lower in summer than other seasons, probably because factors outside houses have a predominant effect on the personal exposure pattern. Adjusted for climate variation, significant factors influencing personal exposure were indoor pollution, total time spent in the polluted city center, and means of transportation used.

In contrast to other studies, we found no differences among urban BTX ratios during the measurement periods, but there were differences in personal exposure and especially in indoor concentrations (see additional file 1, Table 3) [¹⁴,¹⁸,²⁴]. This finding might be explained by the putative effects of indoor sources. The correlation between toluene and xylene was quite good (coefficient 0.59?0.86), similar to those reported in other studies [²³,²⁹]. Comparing the factors that influence exposure to benzene, toluene and xylene, it was found that proximity to a gasoline station or a busy road, and a smoker room-mate, had a much greater impact on both personal exposure and indoor residential levels of benzene (though volunteers were instructed to avoid exposure to environmental smoke and gasoline refueling). On the other hand, the factors that had some degree of influence on toluene and/or xylene concentrations, such as heating mode and recent painting, had no impact on benzene levels [¹⁴,¹⁸].

In Athens, Greece, the time spent outdoors during work or leisure is a major contributor to hydrocarbon exposure among non-smoking citizens. Indoor pollution and means of transportation contribute significantly to individual levels. Other factors such recent painting, smoking room-mate and mode of heating might contribute significantly to individual levels of toluene and xylene exposure. Identification of specific groups of people who may be subject to higher exposure is an important step towards preventing possible adverse health effects and a prerequisite for evidence-based intervention.

The author(s) declare that they have no competing interests.

ECA participated in data collection, carried out the statistical analysis and drafted the manuscript. CC participated in the design, coordination of the study, data collection and helped in the revision. AL participated in the design and coordination of the study and helped in the revision. All authors read and approved the final manuscript.

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