Liesel Wagner, age 10, of Miami, Fla., for her question:

WHAT CAUSES HAY FEVER?

Hay fever is an allergy that occurs most often during spring, summer and fall. The most common cause of hay fever in the United States and Canada is the pollen of the ragweed plant. In most areas, ragweed produces pollen in the late summer or early fall.

We all know the symptoms of hay fever: runny, red and itching eyes and a runny, stopped up nose. The nose may also itch and swell.

A hay fever victim also has repeated periods of sneezing and may temporarily lose his sense of smell. His ears may be stopped up as well.

About a third of those who suffer from hay fever develop seasonal asthma. Hay fever also may result in sinusitis, a sinus infection. In addition, it may cause the development of infection or of growths called polyps in the mucous membrane.

In addition to the ragweed, pollen of the Russian thistle is a major cause of hay fever in New Mexico and Utah. Other weeds in all parts of the continent also give problems.

Hay fever can be prevented or made less severe by avoiding the offending pollens. Some communities have tried to eliminate offending weeds. But wind carries pollen great distances and elimination programs have not generally been successful.

Many newspapers and radio and television stations announce the daily pollen index during the summer months. This figure is based on the number of pollen grains in one cubic yard of air.

The Midwestern states have the greatest concentration of pollen in the country. Areas of little ragweed pollen include the Pacific Coast and the southern tip of Florida.

Hay fever has the same symptoms as year round condition called perennial allergic rhinitis. A doctor performs tests to identify the cause of a patient's allergy. If the person has hay fever, the doctor performs tests to discover which pollen, fungus or mold caused it. Most doctors then prescribe drugs called antihistamines which provide relief for some sufferers.

Sometimes, if a patient is extremely sensitive to pollen, a doctor may prescribe hyposensitization. This treatment involves injecting pollen extract into the patient's body at regular intervals, slowly increasing the strength of the concentration of the extract. The injections cause the body to form antibodies to help fight the allergic reaction. In most cases, this therapy helps provide relief from hay fever symptoms.

Many grasses, trees and weeds produce offending pollen. House dust and even animal dandruff bothers some people. About six percent of the people in the United States have hay fever.

Like most allergies, hay fever runs in families. A person can develop the allergy at any age.

Antihistamine drugs for hay fever should be taken only under a doctor's supervision because they can cause dangerous side effects in some persons.

The medical name for hay fever is pollinosis.