In understanding the health risks a sandstorm poses, it helps to look at what the phenomenon entails.

After travelling a long distance, a sandstorm is likely to be comprised of a small portion of small-particled sand. The majority is made up of dust.

Among the particles in the air, matter under the size of 10 microns affects the health most significantly. They can be divided into three types according to their size.

Particles bigger than 10 microns stay only for a short time in the air and quickly drop to the ground. These can't be breathed in and do little harm to our health.

Dust between 5-10 microns can float in the air for a long time. But most of these are blocked by the hair in the nose, cilia and epidermis of the respiratory tract when they pass the nasal cavity, larynx and trachea. They can then be discharged by coughing and sneezing.

Dust between 0.5 and 5 microns can get deep into the lung and bronchial tract and does the most harm. About 75 per cent of the particles that can be breathed into the alveolus are between 0.5 to 3 micron.

Dust smaller than 0.5 micron can be breathed out of the lung and won't stay inside the body.

The skin, nose, eyes, trachea and lungs are the first places to come into contact with dust. A high density of dust in the air might first make people cough, sneeze, have runny noses and force them to cough out phlegm.

These reactions are usually temporary. But sometimes the reactions can be severe, especially when encountered for the first time or from dust of high density. For example, it might lead to a sudden shortness of breath, an aching or tight chest, headache and dizziness, especially among those with asthma, chronic lung problems and heart diseases.

Dust on the skin can block the sweat glands and cause inflammation if not cleaned promptly. Dust in the eyes increases the chances of conjunctivitis.

Dust from 0.5 to 3 microns in size can enter the lung and bronchia and stay there. This can lower the function of the bronchia and alveolus. Research among primary school students living for a long time in a polluted environment found that their immunity system suffered, leading to lower resistance to infections and increased incidence of respiratory tract problems.

Scientists also found that particle matters might be related to lung problems, heart disease and asthma.

Moreover, particles can attach harmful gas, bacteria, lead from exhaust fumes, pollen and aromatic hydrocarbon produced by incomplete burning of coal and petrol. These can cause harm to the body.

Experts offer advice as to what to do when there is a sandstorm:

*If it causes chronic coughing with phlegm, or shortness of breath, a suffocating feeling or a chest pain, one should seek help from medical professionals as soon as possible.

*Avoid outdoor activities. Stay indoors and close the door and windows. Stop physical exercises outdoors, which might increase the harm of particles getting into the lung.

*If you have to go outdoors, wear masks and goggles to protect the eyes, mouth and nose. A piece of cloth won't work for small particles.

*Wear cosmetics, which can help reduce the harm of dust to the skin.

*Use humidifiers indoors. Or spray water in the air and on the floor to reduce dust.

*Drink more water. This is good for the health of the skin and helps discharge harmful elements in the body.

(Beijing Weekend March 22, 2003)