BAR HARBOR — Thousands of mice at the Jackson Laboratory produce 152 gallons of urine every day — and that is no small health concern for employees who develop allergic reactions.

More than half of the laboratory’s employees eventually develop some type of allergic reaction, ranging from red, itchy eyes to more serious problems such as asthma.

Next to repetitive strain injuries, mouse allergies are the most significant occupational health hazard at the world-famous breeding facility for special strains of mice used in medical research.

At any one time, there are about 650,000 mice in Jackson lab.

Carol Hoglund, who has worked at laboratory for six years, started developing allergies about four years ago, but didn’t know she had an allergy problem until she returned from a vacation.

Her eyes started puffing up, becoming red and itchy. She went to the employee health office, where she was told she was allergic to mice.

“I said, `now that makes sense,”‘ she recalls. “Looking back, I felt I always had a cold; I was always stuffed up, whether it was summer, fall, winter, spring, anytime.”

Mouse urine is loaded with a protein that, when inhaled, acts as a potent allergen. The allergen is manufactured primarily in the animal’s kidneys.

Mouse allergies are most prevalent in the lab’s 22 mouse rooms, especially those in which the animals are raised. On average, two employees in these rooms each year develop health problems severe enough to require a transfer to another part of the lab, and a third person has to leave the lab altogether.

The lab has spent more than $1.3 million to improve indoor air quality and reduce employees’ exposure to mouse allergen during the past three years.

The laboratory is also experimenting with different kinds of bedding and cages to eliminate the problem. The federal Food and Drug Administration has approved the use of the mouse urinary protein to help target sensitive employees through a mouse allergy skin-prick test.