Are environmental toxins causing the rise in early-onset cancers? You asked, we researched.

“Our environment is so toxic, from the air we breathe to the products we put on our body, to the food we put in it,” a reader with the online name Time17 wrote. “It’s no wonder people are getting sicker at younger ages than ever. I am no scientist, but I can see the connection.”

Such a connection may seem self-evident, but it’s exceedingly hard to prove, which even those raising alarms about environmental toxins acknowledge.

Cancer takes many years to develop, and it’s nearly impossible to track which chemicals people were exposed to over decades and in what quantities. It’s especially hard to draw causal connections with cancer among people under 50, because while the rates are going up, the numbers are still relatively small.

“You have to have a very large group of people that you follow for a very long time before you have enough to study it,” said Dr. Beate Ritz, professor and vice chair of the epidemiology department at the UCLA School of Public Health. While such studies are starting, they will take decades to complete.

That said, there’s no question that we are exposed to chemicals — many of them known carcinogens — that are found in air, water, and food. These substances could harm our health, but it’s not clear how much exposure is needed to trigger cancer.

“We have limited data about the proportion of the cancers that might be explained by these chemicals,” said Timothy Rebbeck, professor of cancer prevention and epidemiology at the Harvard T.H. Chan School of Public Health. “That isn’t to say that there isn’t risk there. Our measurement and our ability to understand the risk is very poor.”

Rebbeck points out that, despite the increase among those under 50, overall cancer rates have fallen during the past 50 years.

“If these chemicals as an aggregate had a huge impact, would cancer rates be dropping as they have been?” he said.

Here’s a look at some of the substances readers were concerned about.

These are chemicals that mimic, block, or interfere with hormones, and they’re everywhere — in food, air, water, clothing, furniture.

They include bisphenol A, best known as BPA, which is used in making plastics; per- and polyfluoroalkyl substances, or PFAS, the “forever chemicals,” used in firefighting foam, nonstick pans, and textile coatings; and phthalates, which are liquid plasticizers found in hundreds of products including some food packaging, cosmetics, and children’s toys.

These chemicals disrupt hormonal pathways, and can do harm, especially during pregnancy, early life, and puberty.

But do they contribute to cancer? PFAS are associated with cancers of the kidney and testis in people who have been heavily exposed, such as on the job or in the military. What this means for people exposed to small amounts, or for the risk of other cancers, remains unclear. A 2020 review of the studies so far concluded: “Overall, the evidence for an association between cancer and PFAS remains sparse.”

But the absence of evidence does not equate to the absence of risk, and these chemicals are omnipresent in our lives.

Julia Brody, executive director of the Silent Spring Institute, an organization that researches the links between everyday chemicals and women’s health, said studies over the years have found associations between environmental toxins and breast cancer. Such data, she said, is surely relevant to early-onset breast cancer as well.

The institute identified 300 chemicals in the environment that increase estradiol and progesterone, hormones that can promote breast cancer. “I wouldn’t say they cause breast cancer because we don’t know that yet. But they’re definitely a good place to look at,” said Ruthann Rudel, the institute’s director of research.

The institute has also listed chemicals that cause mammary gland tumors in animals, “because that’s also a very good predictor,” she said.

Women born in the 1950s were in the first generation affected by chemicals such as PFAS and phthalates, and their children, exposed in utero and now in their 30s and 40s, face a greater cancer risk as a result, said Carmen Messerlian, assistant professor of environmental reproductive, perinatal, and pediatric epidemiology at Harvard Chan.

“Our environment has been increasingly becoming hostile to our health,” she said. “This generation has been highly exposed. It’s not surprising at all to me that there are higher rates among people under 50.”

Air pollution is clearly unhealthy, but does it play a role in early-onset cancer? “The answer is, we don’t know,” said Joel Schwartz, professor of environmental epidemiology at Harvard Chan. “It’s likely that it’s having a role in causing some early onset cancer. But whether it has a role in increasing the rate, I don’t know.”

Schwartz recently led a study that showed exposure to nitrogen dioxide and fine particulates in air pollution is associated with increases in colorectal and prostate cancers among Medicare recipients — people older than 65.

It’s unknown whether air pollution has the same effect on people under 50. But air quality has improved in recent decades, so it’s likely that people under 50 have had less exposure to the damaging particles, Schwartz said. (Wildfires are starting to reverse that trend, however.)

Still, the mechanisms by which air pollution might promote cancer are clear.

“Air pollution goes into your lungs but it doesn’t necessarily stay in your lungs. Some of the particles are so small that they can pass through the lung right into the capillaries,” Schwartz said. “Higher exposure to particulate air pollution is associated with a lot with a noticeable increase in markers of inflammation all over the body.”

Inflammation, along with other changes, causes mutations in the cells that advance cancer and “can also interfere somewhat in your immune system’s attempt to kill the cancer cell,” Schwartz said.

Ozone, heavy metals, and particulates can lead to oxidative stress, essentially “rusting” proteins and lipids, said Ritz, the UCLA researcher. Worse, chemicals known as polycyclic aromatic hydrocarbons or PAHs can break the DNA within the cell.

Rudel, of the Silent Spring Institute, pointed to a study that found 3,000 new breast cancer cases with each increase in particulates.

Most of the research on pesticide exposure has focused on people, mostly men, who apply the chemicals and thus are exposed to high levels, and these studies have found an association with higher cancer rates.

But a UCLA study published last year looked at more routine exposures. It found higher rates of thyroid cancer among people who lived within 500 meters of a place where any of 10 pesticides were applied. Dr. Avital Harari, the senior author, said the neighbors most likely inhaled the chemicals, but it’s possible it landed on their food or filtered down onto surfaces.

The chemical most consistently linked to cancer was paraquat, a powerful herbicide that can only be used commercially. But an association was also found with glyphosate, a weed killer found in Roundup.

The cancer risks were strongest among people over 65. But young people were also affected. Indeed, Harari, an endocrine surgeon at UCLA Health, initiated the study because when she moved to California, she noticed much more advanced cases of thyroid cancer, and suspected agriculture. “I started treating 20-year-old females with metastatic thyroid cancer,” not a typical age for such severe illness, she said.

Asked whether she believes pesticides are playing a role in the rise in early-onset cancers, Harari said, “I think they are.” Many factors are at play, she said, including obesity and potentially other environmental exposures, but “Pesticides are at least a piece of the puzzle.”

There are steps you can take to reduce your exposure to environmental toxins, even if you can’t avoid them altogether. Messerlian avoids plastic. She packs food in foil or in glass containers, and never uses a nonstick frying pan. When her son orders a take-out pizza, she immediately removes it from the box, which has a coated surface.

If you’re worried about air pollution, buy an air purifier that filters particles, advises Schwartz, who has two of them in his house.

Online resources provide other suggestions. The Silent Spring Institute offers an app, called DetoxMe, that guides you through safer choices. The Environmental Working Group’s Healthy Living app rates 120,000 food and personal care products, and the organization offers an array of consumer guides.

But in the end, you can’t escape all the toxins in the environment, and it’s not clear whether efforts to avoid them will make a difference.

Even Messerlian says “the best thing you can do” is make lifestyle changes: eat fresh fruits and vegetables, exercise, sleep well, and reduce consumption of processed foods.

“We have to tip the balance from the bad stuff,” she said.

Ritz, of UCLA, notes that greens and other vegetables contain antioxidants. “So even if you’re exposed to air pollution that is causing oxidative stress, antioxidants can counter it.”

Felice J. Freyer can be reached at felice.freyer@globe.com. Follow her @felicejfreyer.