Even before she left the scene, Krista Sullivan knew what the evidence would reveal. She spotted the offenders’ tracks, then rounded up the remains. It looked like an open and shut case.

Back at headquarters, she placed the proof on a slide, stained it with dye and studied it under a microscope.

Just as she suspected: Pollen. Everywhere.

After counting the number of grains on the slide, she plugged the total into a formula and reported a“very high level” pollen reading to the National Allergy Bureau.

Sullivan, 33, along with fellow allergy researcher Jenjira Skrei, 26, has been tasked with a special duty. As the only two certified pollen counters in the state, they track the amount and variety of pollen circulating in Minnesota’s air — posting their findings on websites used by allergy doctors and scientists.

The two women, the keepers of the state’s official pollen tally, are especially busy now that Minnesota is nose-deep in the spring allergy season.

While weather reports, websites and mobile apps provide allergy alerts, pollen counters like Sullivan and Skrei are a rare breed. They’ve undergone extensive training by the National Allergy Bureau, and have mastered the science of identifying the dizzying array of pollen grains as small as dust.

It’s tedious work, but Skrei and Sullivan relish it. For them, discovering what exactly is causing so much misery for the millions with seasonal allergies makes every day feel like a whodunit.

“It’s like we’re detectives,” Sullivan said.

The pair wear scrubs and sneakers and sit back-to-back in their shoebox of an office in downtown Minneapolis.

Self-described “science nerds,” they both hold bachelor’s degrees in biology and work for the Clinical Research Institute, the research arm of Allergy and Asthma Specialists, a clinic in the Twin Cities.

Sullivan and Skrei stepped up to take over the pollen counting duties a few years ago after two other researchers left. In doing so, they joined the little-known ranks of certified pollen counters in the United States — a loosely knit group that includes doctors, researchers and others keen on allergy science.

“It’s a very small group of people,” Skrei said.

She and Sullivan take turns counting grains — alternating days for the chore that typically takes an hour.

Springtime keeps them hopping, with tree pollen starting around April 1, followed by grass pollen season around Memorial Day. There’s a break in the action in July, but by the middle of August, ragweed season kicks into high gear.

After such a long and harsh winter, there was speculation that the polar vortex would usher in a sudden spring — causing all the trees to pollinate at once. But the local pollen police, and their boss Mary Anne Elder, say there’s no proof to support such dire predictions.

“Everyone subjectively feels like this is the worst pollen season,” said Elder, research manager at the Clinical Research Institute. “[But] your memory is influenced by the fact that we haven’t had tree pollen for a year. And we just came out of a really tough winter and everyone just wants it to be spring.”

When allergies hit, most people find relief in over-the-counter medicine to combat the constantly runny nose, sneezing and watery eyes. But knowing the day’s pollen count can help allergy sufferers to be more proactive. That’s where Sullivan and Skrei come in: They sniff out the culprits and alert patients before their sneezing starts.

Hot on the allergen trail

First, they must collect the pollen.

A small collection station is set up in Sullivan’s back yard in south Minneapolis. In accordance with the national guidelines, she posted the device about 10 feet in the air, attached to a flagpole that extends from a tripod set up on her deck.

Every morning, she climbs a ladder to reach the station and retrieves the pollen samples for the day. Oddly, she says, none of her neighbors have ever asked her about the device or her early-morning climbing ritual.

“Sometimes I want to tell them that I’m doing something confidential and I can’t talk about it,” she joked.

Once she’s collected the sample, it’s time to identify the perps.

In the office, she goes through the painstaking process of placing the sample on a slide, adding four drops of dye, then viewing it under a microscope, which magnifies the sample so she can clearly see the features of each grain. Then, she counts — and identifies — every single one.

The total count is then posted on the Clinical Research Institute’s website and is available on a special pollen count hot line run by the institute. The counts also are used by the institute to conduct studies to develop new allergy treatments.

Skrei and Sullivan are able to determine not just how much pollen is in the air but what kinds of trees are producing it. Maple, elm, cedar, poplar, ash and others are among the usual suspects in Minnesota. For the more unusual grains, they turn to a binder full of photographs showing the unique characteristics of hundreds of different pollen types.

“The pine pollen is probably the most fun,” Sullivan said. “That one looks like Mickey Mouse. And the maple pollen looks like beach balls. They’re all distinctive.”

Pausing, she adds with a laugh: “The older ones look like cut diamonds to me.”

And ragweed?

“Lots of spikes,” Skrei says.

Accuracy matters

There are many sources for pollen forecasting — especially in the Internet age — but few are as rigorous as these official counters.

Some allergy forecasts are “completely off,” said Dr. Kraig Jacobson, an allergist and immunologist in Oregon who is on the board of the national allergy organization that trains the counters.

There are hundreds of allergy apps now available for smartphone users, for example. One of them, AllergyCast, operated by Zyrtec, issues a disclaimer on its website saying that the information “is provided by third parties for informational purposes only, and does not represent a guarantee of accuracy. … ”

Having certified pollen counters is key to getting accurate results, said Jacobson, who’s been counting pollen for more than 30 years. “In a lot of ways, it’s to maintain quality control,” he said.

One hundred. That was the pollen count Sullivan and Skrei derived for several days in a row. Then, the number spiked.

Through their meticulously detailed detective work, they tallied it at 2,641.

This was a day that would surely tweak the noses of Minnesota allergy sufferers. They immediately posted their findings.

It was all in a day’s work for the pod squad.

“It’s fun!” Skrei said, “ … at least we think so.”