Martin Harper, National Institute for Occupational Safety and Health, Morgantown, WV, US, points out a hazard in the natural environment

The standard reaction to pollution is to blame the polluter - in most cases man and his activities. But what happens when the polluter is the environment? Recently, we have started to view the environment as a source of substances likely to be harmful to human health. A high-profile example of this 'natural pollution' is the contamination of groundwater by arsenic from natural sources in Bangladesh and India. 

Another, less well-known problem is asbestos that occurs naturally in rock. Naturally occurring asbestos (NOA) is the name given to the silicate minerals serpentine and amphibole that, in certain environmental conditions, form 

exceedingly thin crystals in parallel alignment - with the appearance of cotton or silk. The term NOA can also be used to describe different minerals that crystallise in a finely fibrous manner resembling asbestos, and minerals that can produce elongated particles when broken or crushed.

 

Shovelling and raking of gravel containing asbestos can cause asbestos particles to become airborne (David Terpening, US EPA)

 

The most significant risk from NOA is serious illness that may occur through exposure through inhalation. The illnesses include asbestosis - the chronic inflammation of the inside of the lungs (which is a result of exposure to high concentrations of asbestos), lung cancer and mesothelioma - a cancer that occurs in the protective lining that covers most of the body's internal organs. These latter two diseases result from exposure to lower concentrations and are the largest concern for NOA. Not all particles that might find themselves classified as NOA are likely to have the same disease risk, and trying to determine the exact risk from low-level environmental exposures is not easy. So far, adverse health effects as serious and widespread as those related to arsenic in groundwater have not been observed. For the asbestos particles to become airborne - and therefore pose a health risk - the rock or soil normally needs to be disturbed. 

The nature of the soil and local climate are also important, as well as the type of activity causing disturbance. Simple erosion isn't as much of a risk as man's activities - such as mining and quarrying, excavation for construction, farming, landscaping and gardening. People carrying out these activities have the highest risk occupations, but second-hand exposure is also possible for local residents. Exposure can also occur through recreational activities that seriously disturb the soil such as riding motorised vehicles, bicycles, jogging and other sports. The trailing members of a group of riders, cyclists or runners are known to be exposed to much greater quantities than the leaders, and children may suffer higher exposure than adults.

A community may have lived for many generations in an area with NOA without realising it. Such communities may be more comfortable in dealing with the presence of NOA when discovered than people moving into new developments would be - especially those where there is ongoing major construction activity.

Once NOA is identified, risk assessment is required in order to assess the problem, and this can be challenging and hard to communicate effectively when there is not a consensus on the risk.

Once it is known that a region has NOA, the local population must determine whether the risk is tolerable, and this becomes more difficult with increasing uncertainty in the assessment. Risk tolerance is a function of many socio-economic factors, which may involve politics and the law, and the role of government. In each situation where 

communities deal with NOA there is a unique combination of risk assessment and social issues - which include employment, income and property values.  A different combination of the following groups of people get involved in each case - local activists, people with commercial interests, politicians, media, scientific experts, local and national government agencies, and courts of law. Failure to come up with an acceptable solution to the issue of NOA is not pretty; leading to litigation and political and social recriminations, in addition to loss of amenity, reduction of property value, and ultimately the possibility of impaired future health. With all these factors to consider it is clear that this isn't a situation where the one-size-fits-all approach could work, and how each 'newly' contaminated area is dealt with will have to be custom-fitted. 

NOA is an issue of study, debate and concern within the affected areas of the US today, and also for some countries around the Mediterranean, such as Italy and Turkey, where NOA is relatively common. However, the widespread occurrence of these minerals suggests that this matter will become an issue for many other countries in the future.

Read Martin Harper's critical review 'Naturally occurring asbestos' in issue 12, 2008 of Journal of Environmental Monitoring