For the first time this season, tree-pollen levels on Monday morning registered in the "extreme" category, according to Dr. Donald J. Dvorin, the region's official pollen counter.

In his daily blog, Dvorin, an allergist with the Asthma Center, said that even with showers in the forecast, April will end with "a bad week for allergies."

He said that complaints about congestion, sneezing, itchy eyes, and coughing have been pouring into the center as sensitive humans get caught in the crossfire of this annual love fest.

Pollen can travel for hundreds of miles, and it's just a tree's way of spreading its seed; for the allergic, it is just a tree's way of spreading torment.

Most of the pollen measured Monday came from oak trees, a dominant species in the region. In addition, grass pollen was at moderate levels.

The daily tree count came in at 1,636 grains, well above the trigger for "extreme." So where does that number come from?

The count, taken atop a Center City building, is an estimate of the numbers of rains that would pass through a parcel of air about the size of a refrigerator in a 24-hour period.

It is not a number arrived at lightly.

Dvorin, who is the region's certified pollen counter for the National Allergy Bureau, has been doing this for 30 years.

On workdays before he sees patients in his office at Broad and Race Streets, Dvorin climbs three flights of stairs to the rooftop trap that captures the grains.

Pollen is sucked into a coin-slot-size slit inside the trap, which looks like a small satellite with a large fan blade attached.

The pollen becomes attached to a microscope slide coated with an adhesive. Dvorin removes the slide, applies a dye, and places it under the microscope lens. That's how he infers the daily count.

This has been a great day for pollen grains; warm, dry, and a light breeze ideal for flying.

It hasn't been a great day for allergy sufferers.