Asthmatics have airways that are “twitchy” and often are sensitive to specific airborne materials like allergens from cats, dust mites, or plant fragments. Often these folks also are hyper-responsive to nonspecific materials including all sorts of dusts that may trigger “mechanical” receptors in the airways – responding just to the the presence or deposition of a dust particle(s) on the receptor(s) in the throat. The reaction of the airways (asthma reaction) is very much like that from an inhaled irritant like those found in urban air pollution. So with volcanic dust, even small amounts that you may not even see except as a vague haze may be enough to trigger these mechanical receptors and cause airway narrowing and breathing discomfort. How do we minimize an asthma attack in these instances – staying indoors, wearing a dust mask (found in hardware stores) – even if only a surgical mask is available to stop the largest particles – can help those with asthma. Physical exertion such as running when the air is affected would not be wise. If you have an inhaler prescribed to you,keep it handy.

More information on asthma can be found here http://www.epa.gov/asthma/