Overview
Asbestos exposure in non-occupational settings is measured less compared to exposure in occupational settings. The concentration of non-occupational asbestos exposure is typically much lower and more variable than the concentration in occupational settings. However, as observed in the Kubota Shock in Japan, low levels of asbestos exposure are still responsible for a sizable number of mesothelioma cases, which should not be neglected. One study investigated a cluster of mesothelioma cases among residents living near an asbestos manufacturing factory located in Amagasaki (an industrial city in Japan); however, many of them had no history of occupational asbestos exposure.(1) Similar mesothelioma incidence related to non-occupational asbestos exposure was also reported in residents and workers’ families in Italy.(2)
While industrially contaminated areas are a concern for residents’ health (3), the impact of other non-occupational exposure scenarios on the health of the general population should also be considered. For example, the use of asbestos cement pipes results in asbestos fibers contaminating the drinking water, asbestos-containing houses raise concerns of inhaling asbestos fibers during demolition among nearby residents, and natural mining resources raise concerns of inhaling uncovered asbestos fibers through the wind. Given that non-occupational asbestos exposure may persist even in countries that have banned asbestos, the health risks associated with non-occupational asbestos exposure should not be neglected.
Classifications and definitions of non-occupational asbestos exposure
Having consistent terminology for various sources of asbestos exposure is challenging but necessary to facilitate the identification, assessment, and control of non-occupational asbestos exposure. In general, we can divide non-occupational asbestos exposure into three categories according to its pathway(4):
Neighborhood exposure or residential exposure: refers to exposure resulting from living near asbestos factories or mines
Household exposure: refers to exposure to asbestos-containing materials in home structures, mostly during renovation
Domestic exposure or para-occupational- or household-contact exposure: refers to exposure to asbestos fibers brought home by occupationally exposed individuals, mostly from their clothing or hair.
It is sometimes hard to differentiate specific sources of exposure; therefore, some categories are often mixed in epidemiological studies. For example, the first and second categories are often mixed and defined as environmental exposure.
Measurement of non-occupational asbestos exposure
Non-occupational asbestos exposure is not measured as regularly as occupational exposure; however, the latter is monitored mandatorily. In addition, non-occupational exposure is often measured retrospectively, considering the long latency period of mesothelioma. As a result, the primary way to measure non-occupational exposure is through questionnaire surveys to construct the exposure history. Questions are only presented as dichotomized variables or incremental exposure scenarios. An example of the dichotomized variables is residence (yes or no) in an area with naturally occurring asbestos or in a neighborhood with anthropogenic asbestos sources.(5) Examples for the incremental exposure scenarios include distance to specific asbestos sources (e.g., asbestos manufacturing factory) and years of residence.(5)
Other parameters include the number of sources (as a proxy indicator of the intensity of asbestos exposure) and type of asbestos fiber (as a proxy indicator of residential exposure to certain asbestos minerals identified in the geographic location).(5) Table 1 summarizes some common methods for assessing non-occupational asbestos exposure and examples of exposure occasions. It should be noted that people may be exposed to multiple sources of asbestos.(4) Investigating multiple exposure pathways can help estimate the attributable fraction of asbestos-related disease occurrence and develop preventive actions.
Table 1. Methods for assessing non-occupational asbestos exposure and examples of exposure occassions by exposure categories (4,7,8)
Health risks associated with non-occupational asbestos exposure
The most reported health outcome associated with non-occupational asbestos exposure is mesothelioma. Mesothelioma incidence has been considered a good indicator of the epidemic of asbestos related diseases (ARDs). However, the excess risk of mesothelioma associated with non-occupational asbestos exposure is lower than that reported for occupational exposure, possibly due to lower concentrations of non-occupational asbestos exposure by orders of 1,000-100,000 times (compared to occupational exposure).(6) Through systematic reviews and meta-analyses, studies provided estimates of the relative risk of developing mesothelioma associated with non-occupational asbestos exposure.(4, 7, 8)
Previous pooled estimates are summarized in in Table 2. In general, the risk of developing mesothelioma is significantly associated with non-occupational asbestos exposure (compared to those without asbestos exposure), but the pooled estimates varied across studies, ranging from 2.4 (95% confidence interval [CI]: 1.3-4.5) to 8.1 (95% CI: 5.2-13).(4, 7, 8) The numbers represent the probability of developing mesothelioma with a non-occupational asbestos exposure versus the probability of the event occurring without asbestos exposure. For example, the relative risk of 2.4 means those who have reported household exposure to asbestos have 2.4 times the risk of mesothelioma compared to those who do not have household exposure. The heterogeneity of mesothelioma risk in Table 1 could be due to different methods used to measure asbestos exposure, as we described in the previous section, or the type of asbestos fibers. For example, the estimated relative risk for mesothelioma as related to non-occupational amphibole exposure (ranging from 5.3 to 21.0) is higher than that of chrysotile exposure (ranging from 1.5 to 4.0) (see Table 1).(7, 8)
When data on non-occupational asbestos exposure are scarce and difficult to estimate, the male-to-female ratio of mesothelioma is sometimes used as a relatively reliable surrogate to distinguish non-occupational from occupational exposure in a group of people.9 In general, mesothelioma incidence among males is considered a good indicator of occupational asbestos exposure. Because men have more occupational exposure than women, mesothelioma incidence is higher in men than in women. When a male-to-female ratio approaches one, environmental exposure plays a more important role.(9) This can be regarded as one major feature of mesothelioma associated with non-occupational asbestos exposure.(5) Overall, the population-attributable risk with non-occupational asbestos exposure was 20% in men and 39% in women.(10)
Mesothelioma incidence is more prevalent in adults aged 50-60 years old but rare in younger subjects. Given the long latency period of mesothelioma, prevention of early exposure is a prerequisite for eliminating the disease burden. Studies have shown that boys and girls who attended schools and lived near an asbestos cement plant later in life have a higher increased risk of mesothelioma.(11-13) Therefore, for mesothelioma cases without a history of occupational exposure, tracing the non-occupational exposure history, including childhood exposure, may help to understand exposure scenarios.
Table 2. Relative risks for mesothelioma by exposure categories and fiber types
Managing non-occupational asbestos exposure
Asbestos has attracted overwhelming attention to its hazards to population health. Regulating asbestos consumption (including production, import, and export) is an effective way to reduce and prevent occupational asbestos exposure.(9) Calls for regulating non-occupational asbestos exposure exist along with the control of occupational exposure. However, sources of non-occupational asbestos exposure are multiple, including the house, nearby factories, contaminated areas (waste disposal, landfill, military barrack), and abandoned mining sites.(14) More importantly, given the nature of non-occupational asbestos exposure scenarios, causal relationships and specific sponsors for non-occupational exposure are hard to identify. Reconstructing childhood exposure scenarios, such as transfer of asbestos fibers from a child’s father and his job, may be challenging. The best way we can eliminate non-occupational asbestos exposure is to stop using asbestos—producing nothing, importing nothing, and exporting nothing. Many international calls for a total asbestos ban have been proposed over the past decades, facilitating countries in moving towards a national total asbestos ban.(15)
Implementing a total asbestos ban is just the first step. Afterwards, the disease burden associated with occupational exposure is expected to reduce and shift to non-occupational exposure. To further reduce the risk from non-occupational asbestos exposure, the second step is the identification of non-occupational exposure sources. Houses, factories, and mines where asbestos might occur and spread should be explored.(9) After identifying potential sources, levels of non-occupational exposure should be estimated. These include, but are not limited to, family members exposed to occupationally exposed people or neighborhood exposure based on residence in proximity to asbestos factories and mining sites.(9)
Third, mesothelioma has a long latency period, but is a deadly disease once it is diagnosed. A national surveillance and registry system of mesothelioma and other asbestos-related diseases capturing cases, in a timely manner, is important to facilitate many unanswered scientific and legislative needs and uncover non-occupational asbestos scenarios and disease incidence and mortality.(9) For example, after the Kubota Shock, the Japanese government instituted the “Asbestos Victims Relief Law” to cover compensation for non-occupational cases. Studies about the Kubota victims indicated that the standardized mortality ratios of mesothelioma for residents living within 1.5km from the Kubota plant were significantly high.(16) Such epidemiological reports help establish the relationship between exposure and disease and further help victims get compensation.
Non-occupational asbestos exposure poses a hidden health risk to the general public. Academic research and international experience-sharing can contribute to the understanding of non-occupational exposure scenarios, the measurement of exposure level, and management of health risks. For countries that have banned asbestos, we may now just be beginning to see the effects of the asbestos ban. Countries that have not implemented a total asbestos ban can still start doing so.(9)
