In some eruptions, ash particles can be so fine that they are breathed deep into the lungs. With high exposure, even healthy individuals will experience chest discomfort with increased coughing and irritation. Common short-term symptoms include:
While these short-term effects are generally not considered harmful for people without pre-existing respiratory conditions, we recommend that people take steps to minimise their exposure to breathing airborne ash (see Protecting Against Ash).
Penetration of ash particles into the respiratory tract is largely dependent on particle size. Larger particles (10-100 µm diameter) lodge in the upper airways, while those in the 4-10 µm size range deposit in the trachea and bronchial tubes. Very fine (< 4 µm diameter) particles may penetrate deeper into the lungs, into the alveolar region.
Deposition of relatively coarse particles in the upper airways is primarily associated with symptoms such as irritation of the nose and throat. Deposition of smaller particles in the thoracic region (bronchial tubes and bronchioles) is thought to be associated with acute disease outcomes such as exacerbation of asthma and bronchitis. Very fine particles are termed 'respirable' and may penetrate into the deep lungs where chronic, particle-related respiratory diseases, such as silicosis, are activated.
The most hazardous eruptions are those generating fine-grained ash with a high content of free crystalline silica, as this mineral has the potential to cause silicosis (a chronic lung disease resulting in scarring damage to the lungs and impairment of their function). Silicosis is primarily an occupational disease associated with occupations such as stone-cutting, road and building construction and quarrying.
Some volcanoes mass-produce crystalline silica in lava domes. These are viscous lava piles which grow within volcanic craters and are prone to collapse, generating airborne fine-grained ash rich in free crystalline silica. At Soufrière Hills volcano, Montserrat, West Indies, an eruption began in 1995 and was intermittently active until 2010. This eruption generated dome collapse ash composed of up to 25 wt.% crystalline silica, prompting the UK government to implement controls to minimise population exposure. Comprehensive studies of population exposure to respirable crystalline silica have suggested that the majority of the population are not exposed to sufficiently high airborne concentrations to be at risk of developing silicosis, but a smaller group of individuals (such as outdoor workers) may be at risk of developing mild silicosis.
To date, no longer-term diseases such as silicosis have been attributed to exposure to volcanic ash, although this may be due to inadequate case collection.
It is important to determine levels of free crystalline silica in bulk ash samples after a heavy ashfall, using reliable methods. Particular care must be taken by agencies conducting and reporting on analyses to avoid any confusion between free crystalline silica (where the individual minerals cristobalite, quartz and tridymite are quantified) and total silica content (commonly used to quantify the bulk composition of ash). Within days of the 1980 eruption of Mt St Helens, there were reports in the media that the Mt St Helens ash contained 60 percent or more free crystalline silica – far greater than the actual 3 to 7 percent in the respirable size fraction. This misinformation occurred because of a misunderstanding of the difference between free and total silica.
A further difficulty with this analysis is in differentiating between the peaks (which may overlap) of cristobalite and feldspar. We recommend contacting the IVHHN for advice if in any doubt.
In the event of prolonged population exposure to airborne respirable ashfall (for example, if the eruption is long-lived) it may be necessary for public health officials to conduct more detailed studies on population exposure by using cyclone air samplers to collect samples of airborne respirable dust. These can then be compared to occupational and environmental exposure limits. A commonly-used occupational exposure standard for cristobalite is a limit of 50 µg/m3 of air as a time-weighted 8-hour average, to reflect a standard working day (U.S. National Institute for Occupational Safety and Health, NIOSH). Similar standards exist in many other countries. Environmental exposure limits can be calculated from the occupational standards.