These muscles are classed as 'smooth' muscle. This means they are not under voluntary control, like the muscles of our legs and arms, but instead respond to circulating hormones and compounds released locally by damaged or inflamed tissue. Many drugs will cause changes in smooth muscle without any effects on our voluntary muscles. Smooth muscle contraction will narrow airways and can also constrict arteries and many other tubes in the body. It is thought to exacerbate many respiratory diseases, including asthma.
Sulfur dioxide is produced whenever sulfur-containing compounds are burnt.
Its commonest source in Australia is power-stations burning coal containing slight sulfur impurities. A stimulus that causes asthma symptoms or an attack. Triggers include irritants such as fumes, cigarette smoke, allergens such as house dust mite or moulds, viral respiratory tract infections, and exercise. Not every asthmatic responds to every trigger. And not every asthmatic responds to the same trigger in the same way on each exposure.