The rise and rise of asthma

Key text

What is asthma?

Whenever we take a breath, we take air into our lungs through a complicated system of tubes, called airways. These airways empty the exhaust gases from our lungs and resupply them with oxygen-rich fresh air. Asthma is a condition where the airway walls become inflamed. The airways become narrower than usual and partially blocked, and may eventually become permanently damaged.

Asthma is a long-lasting problem – doctors call it a chronic disease. Sufferers can usually lead relatively normal lives (although they may be a bit breathless and have a frequent cough). Once someone has asthma, symptoms can be set off or made worse by triggers, leading to an asthma attack.

People suffering an attack will have some or all of these symptoms:

• difficulty breathing, or feeling out of breath for no reason;
• wheezing, especially when breathing out;
• tightness in the chest;
• coughing.

During an attack, the airways narrow because the muscles in their walls are squeezing them. This is called bronchoconstriction. In addition, more and thicker mucus is secreted into the airways, and their inner lining becomes inflamed and over-sensitive to any irritants. A severe asthma attack, if not treated in time, can prove fatal.

Causes of asthma

It is hard to know exactly what causes airway inflammation and asthma. Genetic factors play a part. Some people have an underlying predisposition. There is strong evidence that exposure to allergens in early life increases their risk of developing asthma. Asthma is also more likely to occur in someone with relatives who suffer from some form of allergy. Asthma can also be caused by exposure to some occupational sensitisers.

What triggers asthma symptoms?

Once the airways are inflamed, they become twitchy and oversensitive to different triggers, such as:

• allergens;
• polluted air, including cigarette smoke;
• infections of the airways (eg, colds);
• cold air;
• exercise;
• strong emotions.

But it is not always clear what triggers an asthma attack.

Types of asthma

Not all asthma is the same. There are two general types, sometimes called extrinsic (allergic) and intrinsic (non-allergic). About two-thirds of asthmatics suffer from allergic asthma and one-third suffer from non-allergic asthma.

Both types of asthma can be made worse by polluted air, even if it is not a trigger. This is especially true if the air contains sulfur dioxide, nitrogen oxides and ozone. These gases irritate the airways (as do many components in cigarette smoke), worsening an asthmatic’s condition.

Other factors also play a part in asthma. Some studies have suggested that the way we breathe and the depth of our breathing may affect the degree of tightness of the muscles that surround the airways.

Prevention and management

Unfortunately, there is as yet no real cure for asthma. The drugs used to treat it either prevent the occurrence of an attack, or relieve the symptoms of an attack (Box 1: Treatment options). When asthma is controlled by medication, exercise can be beneficial rather than acting as a trigger. Several top athletes are asthmatics.

Incidence in Australia

Asthma has been on the rise in Australia for many years. We have the dubious honour of being one of the world’s asthma hot spots. About 10 per cent of Australians have a problem with the disease, and probably about 20 per cent of children suffer from an asthma attack at some stage. Asthma ranks among the top ten reasons for visiting a doctor.

There are many theories about why the incidence of asthma is so high here. Ideas range from changes in asthma diagnosis (conditions that used to be classified as something else are now diagnosed as asthma) to the occurrence of house dust mites or large quantities of pollen.

Most Australian cities are situated in warm, humid areas – ideal conditions for house dust mites to thrive. The mites are so small that they are invisible to the unaided eye, but they breed in dust, carpets, mattresses and pillows. The mites are harmless, but when their droppings are inhaled they can trigger a reaction that makes some people sneeze and causes asthma in others.

But house dust mites are not the whole story. The rates of asthma around the world do not follow the known extent of dust mite infestations. Asthma rates are lower in less-developed countries and one authority suggests that the reduction in childhood respiratory diseases in our medically efficient world may cause the immune system in the lungs and airways to over-react when exposed to substances that it should tolerate.

Australian research into asthma

Throughout Australia, doctors and scientists are studying the causes and treatment of asthma (Box 2: Australian research).

Box 1. Treatment options

Preventers can stop the airways becoming obstructed. They do this by preventing the inner lining from becoming inflamed and from producing too much mucus. These medicines are therefore called anti-inflammatory agents. Most are corticosteroids (or steroids for short). They must be used carefully because they can cause side-effects. Another type of preventer called cromolyn (or disodium cromoglycate) stops the cells lining the airways from reacting to allergens.

Relievers are a different class of drug. They are designed to relieve the symptoms once an attack is underway. They can also be used before exercise to prevent exercise-induced asthma. The majority are called bronchodilators because they dilate the airways. They do this by making the muscles around the airways relax. There are many different types. Most are inhaled so that they reach the airways immediately.

Peak-flow meters

The peak-flow meter is a way of measuring how the airways are performing. If the peak-flow reading is lower than usual, it is a sign that the airways are becoming partly blocked. In this way the asthmatic can monitor whether extra medication or a visit to the doctor is necessary.

Careful and regular use of the prescribed medication is important in asthma. In 1994, 825 Australians died from the disease, but many deaths can be prevented. Of course, an important part of asthma management is to avoid situations that are known to bring on attacks wherever possible. That is the best medicine of all.

Related sites

• Asthma Management Handbook 2002 (National Asthma Council Australia)

• Asthma – prevention of wheezing attacks (ABC radio's The Health Report, 7 September 1998)

• Route of breathing in asthma (ABC radio's The Health Report, 7 September 1998)

• Can asthma be cured? (Ask the experts, Scientific American)

Box 2. Australian research

One of the great mysteries of the allergic type of asthma – or indeed of any allergic condition – is why the body makes such a fuss over totally harmless airborne particles. We know that what we call allergens are simply materials mistaken for invading microbes or their products. It’s a costly mistake: the entire immune system is put on red alert to counter a non-existent threat, and the result is the gamut of symptoms that sufferers must endure. But why? Grass pollen or mite faeces should not be genuine problems for the human body; after all, the majority of people don’t react to them at all.

Eosinophils respond to allergens in asthmatics

Researchers know that the chief culprit in causing much of the damage in an asthma attack is a fairly rare type of white blood cell called the eosinophil. Eosinophils are designed to be the ‘heavy force’ of the immune system, being equipped to deal with occasional 'big' invaders such as worms. It seems quite bizarre, but in asthmatics these cells are unleashing their strong destructive power onto the patient’s uninfected tissue without any provocation.

Perth research on interleukins

Why and how are the eosinophils brought into the airways and lungs by allergens? In 1985, a team in London discovered that interleukin (an immune system molecule) was controlling eosinophil production and decided to study it. The head of the team was Professor Colin Sanderson. When he came to Perth in 1993, the project moved with him. Research showed that there were actually two chemical messengers involved in eosinophil control. These are now called interleukin-4 (IL-4) and interleukin-5 (IL-5). (The latter was what Colin Sanderson first identified.) These signalling molecules are secreted into the bloodstream by other cells of the immune system (T cells), and can attract eosinophils to the area. If we could block the release of the interleukins, it might stop the accumulation of eosinophils and many of the resulting symptoms of an asthma attack.

The researchers in Perth are finding out what controls the production of IL-5 itself. The aim is to see if there are differences in the regulation of its production between T cells from normal and asthmatic individuals. Preliminary findings suggests that there are differences.

Canberra researchers use genetically engineered mice

At the Australian National University in Canberra, Mr Simon Hogan, a PhD student in Dr Foster’s team, has developed a ‘mouse model’ of allergic asthma. From these asthmatic mice, Dr Klaus Matthaei and Professor Ian Young, also at the John Curtin School, genetically engineered varieties of mice that lacked the genes for expression of either IL-4 or IL-5. When tested with the usual triggers, it was found that the asthmatic mice that couldn’t make IL-5 didn’t have any eosinophils in their airways and lung tissue, and they showed none of the contraction of the smooth muscle of the airways that characterises a full-blown asthma attack. Although they had some types of white blood cell accumulating, they were virtually symptom-free. But the mice that could not make IL-4 had asthma symptoms, which suggested that this chemical messenger is not as important for controlling eosinophils as IL-5.

In a further refinement, designed to prove IL-5’s involvement once and for all, colleague Dr Alistair Ramsay was asked to build a genetically engineered virus to carry the IL-5 gene back into the lungs of the mice lacking IL-5. When the mice were infected with the virus, and Il-5 was synthesised, an allergic trigger produced a full asthma response in mice that previously had produced none when presented with an identical trigger.

Applications of the research

The knowledge from this work gives us a way out of the asthma maze. Blocking the release of IL-5 or inhibiting its effects – such as through a drug that binds to it and inactivates it – could be extremely useful.

The Perth team is now seeking ways of testing for substances that could be used to inhibit IL-5 production or to inactivate circulating Il-5. The Australian pharmaceutical company AMRAD, in Melbourne, will carry out the process of screening likely candidates.

Related sites

• The asthma puzzle (Australia Advances, CSIRO) (requires QuickTime)

• Cytokine Molecular Biology and Signalling Group (John Curtin School of Medical Research, Canberra)

• The modulation of allergic airways disease by Interleukin-4 and -5: Studies using cytokine deficient mice (McMaster University, Canada)

Activities

• Behind the News (Australian Broadcasting Corporation)
  • Asthma (5 April 2005) – looks at what causes asthma and how it is treated. Activity sheets are available.

• Advanced Technology Environmental Education Center, USA
• Environmental risk – asthma assessment – students research what triggers asthma, identify hazards in their homes and make a plan to minimize their exposure to triggers.
• Association of the British Pharmaceutical Industry (UK)
• Breathing and asthma – provides information and then asks a series of questions about the respiratory system and asthma.
• National Heart, Lung, and Blood Institute, National Institutes of Health, USA
  • Asthma awareness – provides a list of activities about asthma.

Activity 1. Structure and function of the human breathing system

1. List the main organs or components that make up the human breathing system.

2. Describe the function of each organ or component.

3. Name a structural feature of each organ or component that helps it carry out its functions.

4. During an asthma attack, which components of the respiratory system are affected? How do these changes alter the function of the breathing system?

Teachers notes

1. The main components of the breathing system are:
   
   • nasal passages;
     
   • pharynx;
     
   • larynx (voice box);
     
   • trachea (wind pipe);
     
   • bronchi;
     
   • bronchioles (lungs);
     
   • alveoli (airsacs);
     
   • capillaries with red blood cells;
     
   • thoracic cavity (including diaphragm, ribs and intercostal muscles).

2. The functions of each component are:
   • nasal passages: warm, moisten and filter air;
     
   • pharynx: channels air to lungs and food and water to the stomach;
     
   • larynx: formation of sounds;
     
   • trachea: passage-way for air (windpipe from throat to lungs);
     
   • bronchi and bronchioles: air passages;
     
   • alveoli: gas exchange (especially carbon dioxide and oxygen);
     
   • capillaries with red blood cells: gas exchange;
     
   • thoracic cavity and diaphragm: ventilation (altering the volume of the chest cavity which changes the air pressure in the lungs).

3. Structural features of components that aid them in functioning effectively:
   • Nasal passages: hairs filter large particles; an abundance of capillaries in the nasal passages help to warm the incoming air.

   • Pharynx: gland cells produce mucus to trap particles.

   • Larynx: vocal cords (elastic ligaments) vibrate when air is directed against them and make sounds. The pitch of the sound is controlled by muscles changing the tension of the cords.

   • Trachea: has reinforcing rings of cartilage to protect the airway in the neck.

   • Bronchi, bronchioles: smooth muscle in the walls of these tubes can relax and contract. Relaxation dilates the lumen of bronchioles producing a larger air passage.

   • Alveoli: these sacs have a lining of thin flattened cells and are surrounded by capillaries facilitating gas exchange between the breathing system and the blood stream.

   • Capillaries and red blood cells: capillaries have thin walls to enable gas exchange. Red blood cells contain haemoglobin that picks up oxygen.

   • Thoracic cavity and diaphragm: the muscle fibres of the diaphragm, and those connected to the ribs, expand and contract to change the volume of the chest cavity.

4. During an asthma attack the smooth muscle of the bronchioles contract. This constricts the air passage and causes breathing difficulties.

Activity 2. Measure the volume of air you can exhale

During an asthma attack the smooth muscle lining the air passages of the lungs becomes constricted and the volume of the air that can be exhaled is decreased. Using a large plastic bottle, you can approximately measure, the maximum volume of air that can be exhaled during forced breathing.

Materials (for each small group)

• 5-litre (or larger) plastic bottle with lid (bottle should be marked with 100 mL gradations);
  
• half a metre of tubing (rubber or plastic);
  
• sink or large plastic container.

Procedure

1. Fill a sink or large plastic container with water.

2. Fill the plastic bottle to the top with water and put the lid on.

3. Turn the bottle upside down in the sink and take off the lid under water.

4. Insert a length of tubing into the bottle as shown in the diagram.

   

5. Take a deep breath and expel all the air in your lungs through the tubing and into the bottle.

6. Record the volume of air you have forced into the bottle.

7. Refill the bottle and set up the apparatus as before.

8. Relax for a few minutes until your breathing pattern returns to normal.

9. Repeat the experiment twice.

10. Average the values you obtain.

Questions

1. How is oxygen used in your body?

2. When does your body need more oxygen?

3. How is the carbon dioxide that you exhale produced?

Teachers notes

Students suffering from respiratory or heart problems should not act as subjects for this activity.

Make sure students do not become competitive in their exhaling.

This activity should be done in groups of two (or more). While one student is exhaling another is needed to stabilise the bottle. Provide a new length of tubing for each student.

You may want students to calculate a class average and determine the standard deviation. Students could compare the average volume exhaled by females with that exhaled by males or they could look for a correlation between volume of air exhaled and height of the student.

Remind students the volume they are measuring is the vital capacity of the lungs which is more than that exhaled during normal breathing (tidal volume) but less than the total lung capacity. (Some air always remains in the lungs because the thorax cannot be completely collapsed.)

Students can also measure the tidal volume of their lungs using the set-up for this activity. They could use the large plastic bottle or a 500-mL cylinder. The plastic bottle will only give a very approximate reading but will show students that the tidal volume is only small compared to the vital capacity.

1. Oxygen is used for aerobic cellular respiration, the process by which sugar molecules are broken down and energy is released.

2. More oxygen is need when more energy is required. More oxygen is required during exercise.

3. During cellular respiration, carbon dioxide is produced. The general equation is:

   oxygen + sugars = carbon dioxide + water + energy.

Activity 3. The effect of exercise on breathing

Exercise can alter the rate of breathing and the carbon dioxide (CO₂ ) content of the exhaled air. This activity will measure the effect of exercise on breathing rate and CO₂ levels in exhaled air.

Materials (for each small group)

• 1 conical flask or small beaker;
  
• 1 100 mL measuring cylinder;
  
• 5 straws;
  
• 1 dropper bottle of sodium hydroxide (chemical formula NaOH);
  
• 1 dropper bottle of phenolphthalein;
  
• 1 stopwatch.

1. Place 100 mL of water in the conical flask and add five drops of phenolphthalein.

2. One member of the team (the subject) should sit quietly for 3 minutes.

3. After the first and third minute take the subject's pulse. (Count the pulse for 15 seconds then multiply by four to determine pulse rate per minute.)

4. After the second minute, test the CO₂ level in exhaled breath by having the subject breath through five straws into the solution in the conical flask for exactly 30 seconds. (The subject should keep the straws together so that as little air as possible escapes between them, and breath in through the nose and out through the straws. There is no need to open the mouth at all. At rest the subject should only take about three breaths in 30 seconds.)

5. Rinse the straws, and put them aside for later.

6. Add one drop of NaOH solution to the flask. Swirl the contents vigorously and observe the flask against a white background. Look for evidence of a pink colour.

7. If no pink colour appears, add another drop of NaOH solution, swirl vigorously and check colour again.

8. Keep a count of the number of drops used.

9. Continue adding drops until the solution in the conical flask turns pink and retains its colour for a least 15 seconds.

10. Record the number of drops used.

11. Wash out the flask and refill with another 100 mL of water and five drops of phenolphthalein.

    You now have two resting pulse measurements and one measurement of CO₂ levels in exhaled air when the subject is at rest.

12. The subject should now run on the spot or do star jumps for 2 minutes, working as hard as possible for the full time.

13. As soon as possible after the exercise stops and while the subject is seated, take a pulse measurement and have the subject breathe out into the conical flask.

14. Repeat the pulse measurement every 2 minutes and the CO₂ measurements at 4 minutes and 8 minutes during the 10 minutes of recovery time. (The subject should remain seated during the recovery period.)

15. Record your results in a table, then graph the data obtained from your subject.

Questions

1. If the average heart pumps about 80 mL of blood with each contraction, calculate the volume of blood pumped by the subject:
   • at rest;
     
   • immediately after vigorous exercise;
     
   • 10 minutes after completing exercise.
     

   (Your answers should be in mL/minute.)

2. Calculate the percentage increase in heart rate from rest to immediately after exercise.

Teachers notes

Students suffering from respiratory or heart problems should not act as subjects for this exercise.

Preparation

• It may be helpful to provide two conical flasks (or beakers) per group of students.

• If the titration is taking a long time, another CO₂ sample can be taken in the second flask.

• Five straws are the maximum needed per subject.

  Since straws vary in diameter, you should experiment with yours to determine how many are needed to give an even delivery of exhaled air into the phenolphthalein solution. (Using multiple straws to exhale through minimises bubbling.)

• Ask subjects to practise breathing out through the straws before beginning the activity.

• If you have more than one subject per group, you will need a new set of straws for each new subject.

• Phenolphthalein solution: dissolve 1 gram of solid phenolphthalein in 500 mL methylated spirits.

• NaOH solution: 0.01 M (0.4 grams NaOH made to 1 litre with water).

• Students should measure the pulse rate by using the tips of the fingers just on the outside of the major vein on the thumb side of the front side of the subject's wrist. Ask students to practise this before beginning the exercise.

Summary of data collection

At the end of the experiment students should have:

• heart rate measurements taken 3 minutes before exercise started and 1 minute before exercise started;

• heart rate measurements taken at 2-minute intervals during a 10-minute recovery period;

• CO₂ measurements taken 2 minutes before exercise started;

• CO₂ measurements taken at 4-minute intervals during an 8-minute recovery period.

CO₂ concentration is measured by titration. When CO₂ dissolves in water it forms a 'weak' acid (carbonic acid). By measuring the amount of NaOH needed to neutralise this acidic solution, a relative measure of the amount of CO₂ is obtained – the more NaOH needed, the more CO₂ has been exhaled.

Activity 4. How an asthma attack causes breathing difficulties

• Explain why this closes off the smaller airways and causes breathing difficulties.

Teachers notes

The large airways to the lungs are supported by cartilage. However, the smallest airways (bronchioles) which lead to the air sacs (alveoli) are supported only by smooth muscle. When these muscles tighten or spasm, this makes the opening of the airways much narrower than usual so it is hard to move air in and out of the air sacs. In addition to the effect of the muscles tightening, the thin linings inside the airways swell and thick mucous plugs the airways. The symptoms experienced by an asthma sufferer are: breathlessness, wheezing, chest tightness and coughing.

Further reading

Useful sites

A wide range of information for patients to help them understand and manage their asthma.
http://www.asthmansw.org.au/content.cfm?id=451&searchvalue=about%20asthma

National Asthma Council Australia

Up-to-date information about asthma for the general public, patients and health professionals. The Asthma Management Handbook is available on-line at this site.
http://www.nationalasthma.org.au

The Lung Net: Learn about lung health (Australian Lung Foundation)

An excellent collection of fact sheets and illustrations about respiratory diseases and lung function. The fact sheets relating directly to lung function and asthma are: 'The lungs - an overview of how they work'; 'Asthma and scuba diving'; 'Asthma in exercise and sport'; 'Controlling adult asthma'; 'Controlling childhood asthma'; 'Occupational asthma'; 'Corticosteroid therapy in respiratory disorders'; and 'Lung function tests'.
http://www.lungnet.com.au/education/learn-health.html

Australian Broadcasting Corporation (transcripts)

• Tomatoes fight asthma (Catalyst, 12 October 2006)
  Reports on two Australian studies on the prevention of asthma and a new vaccine.
  http://www.abc.net.au/catalyst/stories/s1763143.htm

• Asthma (Catalyst, 17 March 2005)
  Australia has one of the highest incidences of asthma in the world and it's on the increase. This is because current treatments, while vital, only alleviate the symptoms.
  http://www.abc.net.au/catalyst/stories/s1325779.htm

• Treatment of children with asthma (The Health Report, 6 November 2000)
  Discusses studies that compare various preventative medications for asthma.
  http://abc.net.au/rn/talks/8.30/helthrpt/stories/s209191.htm

Asthma overview (British Broadcasting Corporation, UK)

Provides information about the symptoms, causes, common triggers and treatment of asthma.
http://www.bbc.co.uk/health/conditions/asthma/

Asthma (Health System, University of Virginia, USA)

An explanation, with diagrams (and sounds), of the human airways and what happens during an asthma attack.
http://www.healthsystem.virginia.edu/internet/pediatrics/patients/Tutorials/Asthma/home.cfm

How your lungs work (How Stuff Works, USA)

Describes how you breathe, where the air goes, how breathing is controlled and diseases that affect your lungs.
http://www.howstuffworks.com/lung.htm

Glossary

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).