For more than two decades, Stephanie Berentsen has had more than her share of allergies' misery: sneezing, severely itchy eyes and nose and the attendant complications of sinus and bronchial infections.

Grass, most trees, weeds, dust mites, dogs and cats. She reacts to them all. She is a patient for all seasons.

Berentsen, 44, is part of the wave of an increase in allergies that has built through the years.

Experts estimate that 40 million to 50 million people in the United States suffer from allergies. Environmental allergies are among the worst, and scientists don't know for sure the causes of the increase.

Many allergists believe the "hygiene hypothesis" may help explain it: Unlike many years ago, children are exposed to fewer germs early in life because homes are cleaner, families are smaller and farm animals aren't around generally. Thus, children's inexperienced immune systems over time may overreact to harmless substances such as pollen and animal dander.

Another theory points to energy-conserving, tightly insulated homes that seal in dust and mold. Many scientists believe an increase in environmental pollutants, especially diesel exhaust particulates, may contribute to the increase. Even some urban landscapers may be part of the problem: They plant male trees that have plenty of pollen but not many seeds to clean up.

Whatever the causes, "it's definitely been proven by population analyses that allergies are increasing. Surveys show it," said Dr. Leonard Altman, an allergist at Northwest Asthma & Allergy Center in Seattle.

Allergies are set off when a "foreign invader" such as pollen, pet dander, mold spores or pieces of dust mites enters the body by inhalation or skin contact. Antibodies called IgE (immunoglobulin E) spring into action and alert mast cells of the nose or lungs to release histamine and other chemicals to deal with the invader.

These substances, in turn, cause blood vessels of the nose to dilate and release blood serum. The serum inflames the mucous membranes and causes a drippy, itchy nose and eyes or sets off asthma symptoms in the lungs.