Summary

Activities involved in the development, securement or reclamation of sites contaminated by asbestos can lead to suspension of fibres in the air. However, the concentration of airborne asbestos fibres depends on a number of parameters, and knowledge of these can affect the choice of occupational prevention or protection measures.

The purpose of this study was to draft a state-of-the-science report on the relation between airborne and soil concentrations of asbestos fibres. To this end, a literature search was performed using keywords, including asbestos together with “soil” and “air.” Studies of real as well as simulated situations were considered, as were experimental studies. A total of 241 publications were identified, but only five were selected for our report, two field studies and three experimental studies.

In sum, the moisture and asbestos content of the soil and the type of activities performed at the site (amount of soil mixing) seem to be the factors with the biggest impact on release of airborne fibres. Other factors of lesser importance that can also have an impact include the nature of the soil, the type of asbestos and the distance from the source of emission.  Last, weather conditions (rain, wind and sunshine), plant cover and the friability of the materials can have a smaller impact on emission of airborne asbestos fibres.

More specifically, the field studies showed that the level of soil disturbance is a key parameter in the airborne suspension of asbestos fibres from contaminated soil. Thus, during the repair of a road covered with asbestos-contaminated gravel (less than 0.5% tremolite-actinolite), the operators of the graders and the inspectors were most exposed, averaging 0.276 f/cm³ and 0.260 f/cm³ respectively, whereas exposure of the flagmen and the road roller operators was not as great, 0.1 f/cm³. In addition, fibre concentrations in samples collected at individual workstations during different types of work (raking, turning and trench digging with a backhoe) on soil in which the distribution of asbestos was not homogeneous (from undetected to 3% asbestos) ranged from undetected to 0.25 f/cm³. Agitated in a test chamber under worst-case scenario conditions, these same soil samples yielded airborne fibre concentrations that sometimes exceeded 10 f/cm³.

Distance from the source of fibre release can also affect the concentration of airborne asbestos fibres. For example, with soil containing 10% asbestos, fibre concentrations near the source ranged from 0.01 to 0.1 f/cm³ and diminished with distance, dropping to less than 0.001 f/cm³ at a distance of more than 100 m from the source, that is, a drop-off factor of 10 to 100.

The experimental studies, conducted in a test chamber using samples in which the different parameters—asbestos concentration, nature of the soil (sand, clay, intermediate), type of asbestos (amosite, chrysotile, crocidolite)—were controlled, demonstrated correlations between the properties of the soil and the generation of airborne fibres.

For example, soils with a higher asbestos content (1%)  generated concentrations of airborne fibres that were 100  times higher than soils with a lower asbestos content (0.001%)—10.8 and 0.11 f/cm³ respectively. Sandy soils generated higher concentrations of airborne asbestos fibres, whereas clay released the lowest concentrations (0.07 – 15.9 f/cm³ and 0.13 – 6.6 f/cm³ respectively). Soils containing chrysotile released the lowest airborne fibre concentrations (0.06 to 6.1 f/cm³), followed by soil containing amosite (0.1 to 12.6 f/cm³) and then soil containing crocidolite (0.17 to 13.8 f/cm³). Airborne asbestos fibre levels depended more on the asbestos content of the soil than on the type of asbestos or the nature of the soil. However, caution must be exercised in interpreting the results of these experimental studies, often conducted under worst-case scenario conditions, and care must be taken not to generalize or extrapolate by applying the conclusions to real situations.

Of all the parameters that can reduce the concentration of airborne fibres released, watering of the soil has the biggest impact. In general, for all types of soil and asbestos studied, even a 5 to 10% increase in soil moisture content has a major effect in reducing airborne fibre concentrations, whereas a 50% increase in the water content of the soil can cause airborne fibre concentrations to drop from 5 f/cm³ to 0.01 f/cm³, that is, a 500-fold reduction. In an outdoor environment, the moisture content of soil with a plant cover is already more than 10%, whereas soil without a plant cover tends to dry up more rapidly, especially under hot and sunny conditions, and is more likely to generate airborne fibres.

There is still too little scientific information, however, to identify with any confidence or accuracy the contribution of asbestos-contaminated soil to the concentration of airborne asbestos fibres and hence to occupational exposure. Given the small number of studies, their methodological limitations, the non-comparability of the field research and the experimental studies and the variability of the results, it is difficult to draw decisive conclusions. Given that the moisture content of the soil is the only parameter on which immediate action can be taken to minimize the release of airborne asbestos fibres, it is strongly recommended that soil be watered before any work is undertaken and that the general principles of occupational hygiene be applied to protect workers.