Published by Australian Academy of Science
Glossary | The rise and rise of asthma

airways. A general term for the system of tubes that runs from the back of the mouth and nose into the lungs. The largest is the trachea or windpipe. In the chest, the trachea divides into two smaller tubes called bronchi. Each bronchus then supplies one lung. After entering the lung, the bronchi divide further into narrower tubes called bronchioles and these supply the air sacs of the lung. The airways contain a thin lining on the inside moistened with a little mucus. Barring choking or drowning, from the moment you are born your airways are kept clear and, unless you smoke, fairly clean. Asthma sufferers find that periodically their airways narrow and they can't breathe normally. The smooth muscle is contracting and so narrowing the airway, and at the same time the inner lining becomes inflamed and reddened. It may also swell. Extra mucus is produced, sometimes thicker than usual, and it can partially clog or obstruct the airway.

allergens. An allergen is any substance that triggers an allergic reaction. Common respiratory allergens are grass pollen, mould spores or house dust mite faeces (present in dust); other allergens may affect the skin or the digestive system.

allergic reaction. Allergies are inappropriate reactions of the immune response to substances (allergens) that normally wouldn’t cause any noticeable effects. Most allergic reactions involve the allergen binding on to special immune system cells and causing these cells to release compounds that affect the surrounding tissue. One such compound is histamine. It causes itching and inflammation. Chemicals that block the effect of histamine are called antihistamines, and they are standard allergy medication. However, they are not particularly effective in asthma.

bronchodilators. (Also called 'relievers'.) These are a group of drugs that relax the smooth muscle in the airway walls and hence widen (dilate) the airways. Used to relieve the symptoms of an asthma attack.

chronic. Used to describe a medical condition that continues for a long time, often with little change. A chronic disease, such as asthma, may have acute episodes, when the situation worsens for short periods of time.

eosinophil. A white blood cell that increases in number as a result of certain parasite infections and allergic diseases.

extrinsic. External or a cause coming from outside. In this type of asthma, the cause of an attack is normally the inhalation of an allergen. Extrinsic asthma is more likely than intrinsic to start in childhood, and often the trigger(s) can be identified and dealt with. In extrinsic asthma, the reaction of the airways is like an allergic reaction, and is similar to hayfever and other allergies.

house dust mites. Tiny mites (about one-third of a millimetre long) that feed off human skin flakes and bodily secretions. They colonise houses, especially in warm, humid areas. They tend to live in carpets, mattresses, pillows and soft furnishings. Although quite harmless, their droppings contain substances that are allergens. Exposure to the droppings (invisible to the eye) can cause sneezing, itchy, red eyes or asthma attacks.

inflamed/inflammation. Inflammation is the process that makes living tissue swell, become painful and turn red. Inflamed tissue contains damaged cells and has a higher than normal blood flow through it – that is why it’s red and warm. It is usually ‘infiltrated’ by many cells of the immune system. Compounds released from damaged cells cause fluid and more inflammatory cells to leak out of the blood vessels in the area; this fluid accumulates and may make the tissue swell or block tubes. Inflammation is often associated with infection but it can also be caused by allergic reactions. One of the major inflammatory cells in asthma is the eosinophil, which can damage the airway lining. This can ultimately lead to permanent damage in the airways. Inflammation of the lining of the nose, for example, causes the blocked nose characteristic of colds or of hayfever. Inflammation of the airways occurs in asthma, but is not unique to it.

interleukin. A chemical messenger secreted by cells of the immune system. They act by affecting the behaviour of the rest of the immune system. For example, they may attract immune system cells to an area of the body or they may stimulate the development of some cells of the immune system.

intrinsic. Instrinsic asthma has no clear connection with allergy. It can start at any age. The triggers are usually infection, polluted air, exercise, or cold temperatures, but some attacks occur without any obvious trigger.

nitrogen oxides. Chemical formula NO_x. This covers the gases nitric oxide (chemical formula NO) and nitrogen dioxide (chemical formula NO₂ ). Both can be toxic but nitrogen dioxide is considered to be of most concern for asthmatics. The main source of the gases in urban areas are motor vehicle exhaust and gas cookers and kerosene heaters indoors. The brown haze sometimes seen over cities is mainly nitrogen oxides. These gases are also partly responsible for the generation of ozone, when acted upon by sunlight in the presence of other chemicals. Although air pollution can cause irritating symptoms and increased asthma symptoms in some people, it is unlikely to be an important cause of asthma in Australia.

occupational sensitisers. Chemicals or compounds that causes airway inflammation leading to asthma. These sensitisers occur in particular occupations such as carpentry (eg, western red cedar wood dust is a sensitiser) and commercial spray painting (some duco paints are sensitisers).

ozone. Ozone (O₃ ) is a form of oxygen. It is a colourless gas that has a very pungent odour. It exists naturally at low concentrations in the stratosphere where it absorbs ultraviolet radiation. In the troposphere it exists naturally at extremely low concentrations. But these concentrations increase when sunlight acts on various gases, coming mainly from vehicle exhausts, and ozone then becomes a pollutant in the troposphere. Ozone is a highly corrosive gas and is poisonous to most organisms. At concentrations as low as 0.00001 per cent (or 10 parts per hundred million) it can irritate the membranes lining the nose, throat and airways and can trigger or exacerbate asthma attacks.

smooth muscle. All airways have bronchial smooth muscles in their walls. 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. Smooth muscle relaxation will dilate (widen) these tubes.

sulfur dioxide. Sulfur dioxide (chemical formula SO₂ ) is an acrid-smelling gas that even at low concentrations irritates the membranes of the nose and respiratory system. 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.

trigger. 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. Some triggers, such as allergens, can cause worsening airway inflammation.

white blood cell. (Also known as leucocytes.) White blood cells are the immune system cells. They can be divided into many different categories on the basis of their function and appearance. Many are not found in the blood at all and those that are may have the ability to crawl out of blood vessels, squeezing between the cells of the vessel walls. While some produce antibodies, others produce cocktails of destructive chemicals, others kill virus-infected cells by punching holes in them, and a further class control the entire immune response. For more information see White blood cells (Puget Sound Blood Center, Washington).