In 2016, when 53-year-old Patrick Smyth met with a dozen people at Tully's Coffee in Seattle, their conversations kept getting interrupted. It wasn’t because it was an especially loud or rowdy space. In fact, it wasn’t really the people at all causing the problems.

It was the milk steamer.

Smyth would start talking when inevitably the coffee machine’s steamer would kick on. He remembers them all cringing and halting discussion because they could not think through the noise. And by the third or fourth time, several people simply gave up and left the shop.

“You have to imagine the volume being five times as loud as it would be for a normal person for those of us who are sound sensitive,” Smyth, a finance manager in Seattle, told The Daily Beast.

Like his companions at this coffee shop, Smyth identifies as a Highly Sensitive Person (also known as having Sensory Processing Sensitivity), a natural trait that can cause acute sensitivity to external or internal stimuli. And the meeting was one of the Highly Sensitive Persons of Seattle & Puget Sound meetup group’s many events for its more than 800 members. Started nearly a decade ago, the support group for those who are especially sensitive holds meetings, like the one at the coffee shop, and multiple social events at least once a month, including potlucks, movie nights, natural walks and dinners.

But what does it mean to be a Highly Sensitive Person?

“A highly sensitive person is characterized by deep information processing, high emotional reactivity, increased awareness of environmental subtleties and easy overstimulation,” explained Judith Homberg, principal investigator at the Donders Institute for Brain, Cognition and Behaviour in The Netherlands, and a specialist in sensory processing sensitivity.

And as with any trait, it can mean a lot of different things for a lot of different people. For Smyth, it means he is triggered by sounds, like hammering, sirens or music with especially strong, fast beats.

But bright flashing lights, violent movies, and even loud people can also be problematic. “People who are not really aware of their own volume level, that will pretty much shut me down,” he said. “I just have to stop whatever I’m doing and just sort of go into a semi meditational state when they’re in the room because it just cuts through my head like a buzz saw.”

For Seattle resident Tamra Excell, 46, being an HSP can involve being so impacted by people’s emotions that they actually become her own. It can also mean picking up on a lot of details around her, which she said can be very tiring. Excell gave the example of watching movies by the Japanese director Hayao Miyazaki. “I took in so many details that I found myself becoming increasingly sleepy, and I couldn’t watch more than twenty minutes straight,” she said.

Smyth and Excell are far from alone. Psychologist Elaine Aron, who began studying this phenomenon almost 30 years ago, reports that 15 to 20 percent of the population is highly sensitive to a variety of stimuli—"too many to be a disorder, but not enough to be well understood by the majority of those around you,” she explains on her website.

Aron has since written a handful of books on the topic and created a 27-part yes or no test to help people determine whether they may be highly sensitive. It’s a rather simple test that includes such statements as: “I am easily overwhelmed by things like bright lights, strong smells, coarse fabrics or sirens close by” and “I make it a high priority to arrange my life to avoid upsetting or overwhelming situations.” If one answers affirmatively to more than 14 of these statements, Aron says chances are high a person is highly sensitive.

Aron created the test after interviewing a variety of sensitive individuals, said Homberg. After finding out about themselves and their experiences, she was able to condense the information down to these questions.

“ You have to imagine the volume being five times as loud as it would be for a normal person for those of us who are sound sensitive. ” — Patrick Smyth

It has been used widely in research and has been validated by a variety of researchers, according to Marwa Azab, a psychology and human development professor at California State University in Long Beach. But just like with anything in science, it could be improved, she said. There is now more information on genetics and the brain, and more high-tech technology available. “When science hears the chatter from the public arena about such topics as ‘HSP’, they will use these advanced methods to service a better understanding of the phenomenon,” she said. “This might culminate in updated versions or even new scales that measure HSP tendencies.”

But at this point, the test is the only validated tool to determine an adult’s level of sensitivity, said Francesca Lionetti, a postdoctoral research fellow at Queen Mary University of London’s School of Biological and Chemical Sciences. This can be problematic given that “assessing sensitivity with a questionnaire can increase the risk of biased responses,” she said. Researchers are working to develop an observational method for measuring sensitivity in very young children, which could further advance the understanding of Highly Sensitive People.

Since Aron first started working on this subject, there have been an increasing number of studies published on the topic of high sensitivity. Lionetti cited a 2015 study on the link between sensitivity and a person’s central nervous system. They discovered that “an increased sensitivity to environmental and social stimuli is driven by a more sensitive central nervous system, which perceive and processes information more deeply,” she explained.

There have also been findings published this year in the journal Translational Psychiatry that show there are three types of sensitivity groups in the general population, rather than just two, which Lionetti said could help to clarify each category a bit more. There is ‘high sensitive,’ which is “characterized by higher neuroticism and lower extraversion while being more susceptible to positive mood induction,” according to the report. There is low sensitive, which includes those that are “more extraverted and score lower on neuroticism but also have a lower positive emotional reactivity.” And there is medium sensitivity, which is in between these two groups.

Additional information and research on sensitivity is still needed so that it can be better understood. Homberg said clinicians can misdiagnosis patients or not help them appropriately because they don’t fully understand the trait. “HSP is for instance strongly linked to depression,” she said. “Knowing that you are highly sensitive may allow a better control over environmental influences, and thereby prevention or reduction of depressive symptoms.” They’re not pushing to be in the American Psychiatric Association’s Diagnostic and Statistical Manual of Mental Disorders; they’re simply looking to be understood as different and having those around them understand how these differences might affect them.

It is clear there is a lot more work that needs to be done on this phenomenon, but for now, simply having a term to describe this trait and knowing there are many others who are highly sensitive appears to be helpful. Smyth said meeting with others who have had similar experiences in their daily life makes him feel less alone. “It’s a real eye-opener to go into this situation where there’s 10 other people around this table and then you hear people describe exactly the same sort of thing you experience in everyday life and you thought you were just imagining it,” he said.