Pollen is with you wherever you go. And that means it knows where you’ve been.

Or at least, it’s a dead give-away in the hands of someone who knows what they’re looking at. That someone is Andrew Laurence, one of two forensic pollen scientists—known as palynologists—now working for the US government. These days, he spends most of his time looking at pollen attached to dead bodies and drugs.

Pollen is everywhere this time of year. Right now, in any given city in the mid-latitudes, there are likely to be at least 100,000 pollen grains per cubic meter of air, mostly invisible to the naked eye. Even indoor spaces are likely to be stuffed with roughly 5,000 grains of pollen per cubic meter. Those numbers are likely to go up in more rural places. Each of us, and every object around us, are inundated.

“My allergies are acting up,” Laurence says. “I know, it’s ironic.”

Once pollen makes contact with your clothing or your hair, it will likely never leave. Some pollen have spikes, and many are coated in lipids that act like glue. The pollen count may go down a little with each cycle in a washing machine, but they won’t go away. “You’ll never get it out of your clothing or your hair,” Laurence says. “It’s with you from the day you bought your clothes.”

Andrew Laurence/Chicago Laboratory/US Customs and Border Protection Pollen from Alternanthera, which are flowering plants in the amaranth family.

There are about 380,000 distinct plant species in the world, each with their own unique shape of pollen grain. The grains are rigid geometric cages to house plants’ male sexual matter as it makes its journey away from the parent plant to, it hopes, the female parts of another plant. (Pollen isn’t the plant’s sperm; it is the proto-sperm, in a sense, in that it has the potential to form sperm if it makes contact with the stigma, or female part, of another flower.)

The organic matter of pollen, the proto-sperm cargo, is what makes allergies act up. But what Laurence looks at is the shell. “It’s like an exoskeleton,” Laurence says, and it’s virtually indestructible. “This is how we know what the environment looked like 500 million years ago. That was historically done using pollen.”

Pollen grains are mesmerizing examples of complex architecture at a scale smaller than the width of a human hair. All of them are unique to the plant they came from. And it’s Laurence’s job to tell them apart.

Andrew Laurence/Chicago Laboratory/US Customs and Border Protection Spruce tree pollen.

Laurence’s lab is in Chicago. Border Protection and law enforcement officers typically send drugs seized from the border with Mexico to Laurence and his lab partner, Shannon Ferguson, the only other forensic palynologist employed by the federal government. Ferguson began working for Customs & Border Protection (CBP) as an intern in 2015, and joined the lab full-time in 2017, after earning her PhD.

As the fight against opioid trafficking ramps up, an increasing share of the packages they receive are bricks of fentanyl and its analogues. “They’re wanting to know exactly where they’re coming from,” Laurence says. “I can’t go into specifics, but yes, pollen has revealed new insights.”

In March, CBP published a flier encouraging any state or local law enforcement with more that one kilogram of fentanyl or other opioids on their hands to send them to Laurence and Ferguson so they can figure out the drugs’ origins. The sample could be conveniently shipped “via FedEx, UPS, DHL, or USPS,” they write, and the pollen testing would “fill critical intelligence gaps that need to be addressed in order to determine if the seizures tested to date are outliers or indicative of a wider trend.”

They need a whole kilo of drugs to get enough pollen for a clear signal of origin, or at least 200 pollen grains total, which Laurence and Ferguson painstakingly count and sort by hand under microscopes. The drug sample itself has to be destroyed in the process, to leave only the pollen structures. They eliminate the drugs by bathing them in acids, like hydrochloric acid, which is, as Laurence puts it, “what they use to dissolve bodies on Breaking Bad.”

Andrew Laurence/Chicago Laboratory/US Customs and Border Protection Maple tree pollen.

The palynologists have tested roughly 15 fentanyl samples over the last year and a half. It isn’t Laurence’s main focus yet, he says, but “it’s about to be.” As more opioid cases move through the courts, the drug seizures that sparked them are being prepared for shipment to Chicago. (They tend to have to wait until after legal proceedings are nearing their end, since pollen analysis requires destroying the evidence.) He knows of 34 more fentanyl seizures being prepared for shipment to them now. That’s on top of their year-long backlog. There are always more cases where pollen forensics could help than they have time to analyze.

To figure out what each type of pollen means for an investigation, Laurence has to think through several variables. He consults daily wind patterns on the US National Oceanic and Atmospheric Administration’s website, which can offer some clues. But a lot of it comes down to the characteristics of each plant. Pine, for example, is a wind-pollinated tree, so its pollen has special air bladders to keep it aloft on the lightest breeze. That means pine pollen can show up 100 miles away from the nearest pine tree. That’s when the total abundance of pine pollen grains stuck to sample is vital; if 40% of the pollen on a sample is pine, that means it was in or near a pine forest, which narrows down the geographic options a lot. But one or two percent pine doesn’t mean much. “It confirms you’re on Earth,” Laurence says.

Meanwhile, fire weed, a plant with often purplish flowers, is a very different story. “Fire weed only produces a few grains per season. And they’re really heavy, they don’t travel.” If you find fire weed on drugs or clothes, that means they were almost definitely living in a fire weed ecosystem. Other plants, like agave, are bat pollinated, which means it travels with the bats; the palynologists have to refer to bat flight patterns. Monarch butterflies are another important pollen carrier, and could be dropping pollen anywhere along their migration. “They’re going from Mexico to Canada. If they hit your windshield they discharge their pollen.”

Laurence finds he thinks about the world not according to country borders or state lines, but in terms of vegetation zones. “Plants don’t care about human boundaries,” he says.

Steven Bansbach/US Customs and Border Protection Andrew Laurence in the lab.

Steven Bansbach/US Customs and Border Protection Forensic palynologist Shannon Ferguson looks at pollen grains under the microscope.

Laurence’s investigation work didn’t start with opioids. In 2012, he was working on a PhD about starch reconstruction at Texas A&M University, looking at how starch can be used to identify what ancient cultures ate (starches, like pollen, last a long time). That year he was recruited into CBP through his advisor, Vaughn Bryant, who had been working part-time for the Department of Homeland Security (DHS), looking at pollen samples associated with “persons of interest” in terrorism cases.

“I was looking at samples that had been collected by the intelligence community from places where terrorists were active. There were samples of everything from rugs to cars to people to dwellings to weapons—even bombs,” Bryant told Frontline, CBP’s internal magazine.

CBP, which falls under DHS, was just beginning to explore the idea of incorporating palynology into their investigations. By 2013, Laurence was hired as the agency’s first full-time forensic palynologist, mostly looking at the origins of seized marijuana. In 2014, the National Center for Missing and Exploited Children tapped Laurence to work on child homicide cases.

“Marijuana is more straightforward. It’s grown in one location and shipped out. Homicide is more complicated, because you have ornamentals,” or trees and flowers that people deliberately plant for decoration, but which are not native to the area.

In July of 2015, Laurence was sent baby pants, a blanket, and a bit of hair. The remains of an unidentified toddler had been found in a trash bag on the shore of Deer Island in Boston. The girl, who investigators referred to as “Baby Doe,” was about four years old. Investigators didn’t know who she was or where she came from. But Laurence found evidence of tell-tale ornamentals.

As the Boston Globe reported at the time:

Baby Doe had played among the pines and oaks of New England; she was dusted with traces of privet hedges and cedar-of-Lebanon, which are not native but are often planted in the suburbs. The soot mixed in with the pollen told investigators her surroundings were urban. Somewhere near Boston, they concluded.

The pollen analysis showed investigators the child was local; The girl’s mother and mother’s boyfriend were charged a few months later. The case put Laurence’s work on the map, and proved it could be pivotal in criminal cases. “It’s the gratification of seeing the end result, of having an impact,” he told the Atlantic.

Andrew Laurence/Chicago Laboratory/US Customs and Border Protection Pollen from the Umbellifers family, which includes parsley, carrot, fennel, dill, and parsnip.

Since then, he’s worked on other homicides, plus crystal meth, cocaine, and heroin. A lot of the job is experimentation. There are only about 1,000 forensic palynologists working worldwide, and most of them work for oil companies, who use fossil pollen analysis to help figure out where oil deposits are, and how old they might be (the species of fossil pollen at a specific depth is a signal of the geologic era in which those plants lived). So Laurence and Ferguson have to figure out a lot of techniques for themselves. “My first heroin case was black tar heroin. It was seized inside a fire extinguisher being smuggled across the country. They sent me a log, basically, of crystalline heroin,” Laurence remembers. He began whacking it with a hammer to knock out smaller chunks, to test what acid would be best to dissolve out the drugs. He got pretty good at precise hammer whacks, knocking off just the right 100-gram lumps at a time, he remembers.

Andrew Laurence/Chicago Laboratory/US Customs and Border Protection Pollen from a sweetgum tree.

Once the agency sent him little Tupperware containers full of crystal meth seized in Southern California. Meth production involves several chemical washes, making pollen identification more difficult; one tends to only get pollen from the last stage of production. But it turned out to be vital information for the case. “The samples were loaded in sagebrush. That was pretty cool.” He was able to trace it to Northwestern Mexico, in the chaparral region, where sagebrush grows.

The US is relatively late to come to using pollen as a forensic tool. New Zealand and the UK have been doing it for decades. And while the US only uses palynology as a tool for investigations, in New Zealand, the findings from pollen analysis are admissible in court. But with an era of opioid investigations ramping up, forensic palynology is poised to get its time in the US spotlight.