If you’ve ever had a period — or if you just watched a lot of TV in the 1990s — you probably remember the blue-liquid ads.

In one example from 1995, an actor extols the virtues of Always maxi pads as sapphire-colored fluid dribbles from a glass ampoule onto pristine white fabric. “It protects me better than any regular maxi I’ve tried,” she assures the viewer, though what exactly she needs protection from is left politely unsaid.

The blue-liquid ads were easy to mock — Was that stuff water? Antifreeze? — but they were rooted in something real and enduring: American culture is so uncomfortable with menstrual blood that many people have trouble looking at it and talking about it.

“We’ve seen menstruation and menstrual blood or fluid represented in such a strange, sanitized way,” said Jennifer Weiss-Wolf, author of the book Periods Gone Public: Taking a Stand for Menstrual Equity and the executive director of the Birnbaum Women’s Leadership Center at NYU School of Law. That discomfort has shaped the development of period products like tampons and pads from the very beginning, with real, sometimes serious, implications for everyone who menstruates.

And the blue liquid? It wasn’t just for ads. Early tampons were actually tested with blue-dyed saline solution, rather than actual blood. That means that today, we still don’t know how much menstrual fluid a product like a tampon or pad can hold, making it difficult for ordinary people to know how much they’re bleeding, and even harder for doctors to diagnose conditions like endometriosis that are associated with heavy periods. “We’re very, very behind,” said Alice Lu-Culligan, a pediatrics resident at Boston Children’s Hospital who has studied menstruation.

In the US, periods have historically been treated as shameful and disgusting

The lack of reliable testing for period products is inextricably linked to a problem that potentially puts the health and well-being of millions of Americans at risk: When it comes to understanding menstruation, medical science is woefully lacking. A PubMed search for “menstrual blood” turned up just 400 citations over the last several decades, compared with 10,000 for erectile dysfunction, according to a recent editorial in the journal BMJ Sexual & Reproductive Health. This lack of knowledge and attention has implications for our understanding not just of periods, but of everything from bleeding disorders to pregnancy, experts say.

Menstruation hasn’t always been stigmatized. “In many cultures, it’s seen as sacred,” said Kathryn Clancy, an anthropology professor at the University of Illinois Urbana-Champaign and the author of Period: The Real Story of Menstruation. European societies, however, developed taboos around menstruation in the Middle Ages, and later exported them to the US, influencing the way periods were seen and period products were developed and marketed.

Take tampons. Invented in the 19th century to absorb blood from injuries, they became popular as period products in the 1930s, after Earle Cleveland Haas, a doctor of osteopathy, patented a cardboard applicator to make them easier to insert into the vagina, said Sharra Vostral, a historian and professor of instruction in communication studies at Northwestern University and the author of Toxic Shock: A Social History. When asked if he measured any part of the vagina or cervix during research and development, he said, “I have seen so damn many of them I had an idea. Some are short and some are longer, of course, but that didn’t make any difference.”

That comment was emblematic of the way uteruses, vaginas, and menstruation were treated in American culture — as topics to be avoided whenever possible. In this environment, advertising period products also posed challenges. With even the word “period” considered taboo, advertisers began using blue liquid as a workaround in the 1990s. Unlike red, “blue is uniquely clinical and evokes cleaning products, like bleach or dishwashing liquid, emphasizing a sense of ‘cleanliness’ and hygiene,” JR Thorpe wrote at Bustle in 2017. “It could show absorption and create a sanitary atmosphere instead of reminding people about, well, blood.”

The stigma has extended to the way period products are tested

Advertisers weren’t the only people who had some hesitation around, well, blood. Prior to the 1980s, tampon manufacturers tested their products with a device called a “syngyna” (short for “synthetic vagina), Vostral said. The glass device used a condom to mimic the vagina and saline in place of menstrual fluid. According to one researcher, the saline solution was blue.

In addition to their failure to test their products on real people or real blood, manufacturers had no uniform standards for absorbency. There was no reliable way for a customer to walk into a drugstore and choose the most or least absorbent product. This was an inconvenience for consumers, and soon, it became a health risk, too.

In the 1970s, menstruating people began developing toxic shock syndrome, a condition usually caused by toxins from Staphylococcus aureus bacteria. By 1980, epidemiologists had linked the condition to super-absorbent tampons, which introduced oxygen into the vagina, making it easier for the bacteria to grow. To reduce the risk, the FDA recommended that people use the least absorbent tampons they could. But without consistent testing or labeling, there was no way for people to know which products were actually the least absorbent. In 1982, a task force composed of manufacturers and consumer advocates convened to study the issue and develop such a standard.

One of the advocates was Esther Rome, co-author of the reproductive health sourcebook Our Bodies, Ourselves, who recruited Nancy Reame, a nursing professor who was one of few researchers to actually study menstrual blood. Reame did not trust the tests performed by tampon manufacturers, who seemed ignorant of and disgusted by the actual process of menstruation (“they don’t even understand the functioning of the female human vagina,” a female researcher once told Reame). She and other advocates doubted that saline could be an effective stand-in for menstrual fluid, which does not have a uniform consistency. So she decided to test tampons using expired blood from a hospital instead.

Reame found that some tampons actually absorbed more blood than saline, meaning the manufacturers’ tests had indeed been inaccurate. But amid fighting among the tampon manufacturers, her findings never gained traction. Ultimately, the FDA approved new absorbency standards for tampons, based on experiments with saline. “One thing we learned from the Tampon Task Force is that the materials used in tampons have not been tested adequately,” Rome and co-author Jill Wolhandler wrote in a 1985 New York Times op-ed. “Most have never been studied in the vaginal environment.”

Reame’s experiments were largely ignored, but her conviction that menstrual blood was worthy of study underlies the work that scientists and advocates are doing today. A team of researchers at Oregon Health & Science University recently tested an array of tampons, pads, cups, discs, and period underwear to find out how much actual blood — not watery blue fluid — they could hold. Using expired blood from the university blood bank (real menstrual blood would have been too hard to collect in the quantities they needed), they found that some products absorbed more than their marketing materials advertised. The team’s study, published earlier this year in BMJ Sexual & Reproductive Health, was widely touted as the first to use real blood, though it’s likely Reame’s work that holds that distinction.

Inadequate testing has real implications for people’s health

The menstrual product market has changed a lot in recent years, with the entry of cups, discs, and period underwear. However, the old saline standards still largely haven’t changed, and newer products don’t have any standards at all, said Bethany Samuelson Bannow, the lead author of the Oregon study. That has had implications beyond toxic shock syndrome.

Doctors often rely on menstrual products as a diagnostic tool. To measure heavy menstrual bleeding, which can be a problem in its own right as well as a symptom of conditions like endometriosis, clinicians often ask how many pads or tampons a person uses in a certain time period. But without a reliable measure of how much real blood these products hold, doctors don’t have an accurate idea of how much blood their patients are actually losing. As a result, they may be missing patients who need testing for an underlying condition or who are at risk of problems like anemia.

“It’s a very challenging environment to know how to advocate for ourselves as patients when there is little appetite for these discussions,” Weiss-Wolf said.

Samuelson Bannow, who studies disorders that can cause heavy menstrual bleeding, tested an array of menstrual products to get a better idea of their real-world absorbency. She and her team found that “a lot of the modern products hold a lot of blood,” she said. The product with the highest capacity, the Ziggy Cup menstrual disk, held up to 80 milliliters, which is considered “excessive blood loss for an entire menstrual cycle,” the team wrote.

Their findings show that a lack of reliable testing and standards for menstrual products has had a real impact on patient care. “We’re underdiagnosing heavy menstrual bleeding, because we haven’t realized just how much most of these products hold,” Samuelson Bannow said.

While we know that manufacturers in the 1970s used saline to test their products, it’s not at all clear what they’re using today. Vox reached out to Procter & Gamble (parent company of Tampax), Kimberly-Clark (Kotex), Diva, Knix, and Intimina (which makes the Ziggy Cup), and none responded to inquiries about testing methodology by press time. Despite the fact that these products are used by close to half the American population many times every year, we don’t actually know how, or if, they’re evaluated.

All of this is a symptom of a deeper problem; in a lot of ways, we’re still stuck in the blue-liquid era. Menstruation is “something that we as a society have chosen not to prioritize,” Samuelson Bannow said. “The scientific community is not immune to the effects of sexism and taboo in the culture at large.”

The lack of research on menstruation makes it harder to diagnose and treat endometriosis, fibroids, polycystic ovary syndrome, and infertility, said Lu-Culligan, who has studied the impact of Covid vaccines on menstruation. Indeed, although endometriosis affects one in 10 women in the US, it takes an average of 10 years to get a diagnosis.

For ordinary people, the enduring stigma around menstruation and the lack of standards for menstrual products make it hard to figure out if their bleeding warrants medical attention. People still don’t receive education about what’s normal and abnormal when it comes to periods, Weiss-Wolf said.

Today, however, a growing number of scientists are following in Reame’s footsteps by studying periods and period blood. Some are exploring the possibility of using menstrual blood to test for a variety of illnesses, including HPV, and even for blood sugar monitoring. Samuelson Bannow is interested not only in how much the uterus bleeds, but in how it stops. “We’re always trying to figure out how to stop bleeding,” she said. “We have an organ that literally bleeds every month for 40 years, and we don’t understand how it stops bleeding.”

There’s “a whole new generation of scholars that are not willing to be quiet” about menstruation, Vostral told Vox, “because there’s no reason to be quiet.”

Correction, November 27, 4:30 pm ET: A version of this story that appeared on Apple News had an outdated affiliation for Sharra Vostral, who is a historian and professor of instruction in communication studies at Northwestern University.