Personal exposures to airborne dusts are at best only moderately correlated with ambient particle concentrations measured at central locations that are believed to be representative of a given community. Outdoor particle concentrations vary even within a small area depending on proximity to particle sources such as roads. Indoor particle concentrations are affected by indoor sources such as cooking, tobacco smoke and dust arising from housework, as well as outdoor concentrations and the extent of indoor-outdoor air exchange. In addition to the variable exposure that arises because individuals spend differing periods of time in different indoor and outdoor environments, individuals create their own dust cloud through the activities they undertake. This includes disturbance of deposited dust on surfaces, smoking and cooking. In assessing the health effects of volcanic ash, it is therefore important to have some estimate of the range of probable personal exposures to the ash as well measures of ambient concentrations.
Methods of assessing exposure to volcanic dust are based on those used to measure personal exposure to dust in the workplace. Several types of sampler are available for workplace sampling and these allow for sampling of either the respirable fraction (the fraction that penetrates to the gas exchange region of the lung) or the inhalable fraction (particles that may be deposited anywhere within the respiratory system). Mean exposure concentrations over a shift or a day can be calculated as the sum of exposures for each task (concentration times time as minutes or hours) divided by the total number of minutes or hours in the time period of interest.
The practical difficulties in measuring personal exposure concentrations over a 24 hour period mean that modelling has an important role in exposure assessment for the general population. Different activities or tasks undertaken during the day can be assigned exposure concentrations on the basis of measurement data for the same or similar tasks. Ideally a range should be given for each estimate. Exposure concentrations at night are assumed to be the same as those prevalent in indoor air in the bedroom. If possible, measurement data can be used to establish the indoor/outdoor concentrations of dust. In the absence of measurement data, expert judgement is required to determine whether indoor concentrations are likely to be more or less than outdoor concentrations. Relevant factors will include air exchange, the presence of indoor particle sources such as tobacco smoke and air conditioning. In the absence of better information, it may be necessary to assume that indoor concentrations are approximately equivalent to ambient concentrations measured at a central point. Mean exposure concentrations over 24 hours can then be estimated. The results should be expressed as a mean estimate with some indication of uncertainty. For example, a range of potential exposure concentrations could be quoted.
Modelling is of particular value to the estimation of exposure over a period of months to years. A matrix can be established in which environmental conditions are linked to estimated personal exposure concentrations for groups of different activities. Relevant environmental conditions would include the amount of recent ashfall/quantity of ash in the environment as a defined depth range of unconsolidated ash and the weather in terms of quantity of rainfall and wind strength and direction. A typical daily/weekly activity pattern can then be estimated forindividuals or groups of individuals based on observation and interviews. A spreadsheet is then used to link environmental conditions to activities to estimate mean personal exposure concentrations over the period of interest. This approach has proved very useful in the prediction of the potential risks of silicosis and other respiratory ill health arising from exposure to volcanic ash containing crystalline silica on Montserrat in the West Indies (Searl et al, 2002a and b). We are currently in the process of updating our exposure assessment for Montserrat and predicting the likely levels of exposure over the next few years, in order to inform a new assessment of the long term risks of respiratory ill health on the island.
Download our pamphlets on preparing for ashfall and on the health hazards of ash. They are designed for mass distribution at the onset of new eruptions. They are now avaiable in English, Japanese, French Spanish, Portuguese, Swahili, Indonesian and Icelandic with Italian versions being available shortly. Please see our Pamphlets page for further infomation.