It’s been a bad week for companies trying to fight off lawsuits claiming that their products cause cancer.

A Missouri jury awarded 22 women $4.6 billion in a joint lawsuit against Johnson & Johnson on Thursday, supporting their claim that talcum powder caused their ovarian cancer. And a federal judge in California said Tuesday that cancer victims and their families could present expert testimony linking the main ingredient in the weed killer Roundup to non-Hodgkin's lymphoma.

In both cases, the science is far from certain. Cancer experts disagree, often strongly, on whether either talc or glyphosate, the main ingredient in Roundup, causes cancer in people.

That doesn’t stop the lawsuits. Both Johnson & Johnson and Monsanto, the companies that make the two products, are fighting off multiple claims.

The issue of what causes cancer can be difficult for people to understand. Some causes of cancer are indisputable, said Susan Gapstur, vice president and cancer epidemiologist at the American Cancer Society.

“We have many modifiable risk factors" for cancer, Gapstur told NBC News.

Those include cigarette smoking, excess body weight, alcohol consumption, sun exposure, HPV and other infectious agents, she said. HPV, the human papillomavirus, is the cause of virtually all cases of cervical cancer, as well as many cases of head and neck cancer, penile cancer and anal cancer.

Scientists know this because they can not only see that people infected with the virus are more likely to develop cancer, but they can also find specific damage caused by the virus in tumors.

There’s also strong information about occupational exposures to some chemicals, such as benzene, “when people are exposed to very, very high levels,” Gapstur said. “That information is quite solid and we are quite sure about it.”

But it becomes much trickier to say whether everyday exposure to various household substances causes cancer, even if people are eager to find something or someone to blame for the disease.

The American Cancer Society has devoted a web page to the question of whether baby powders or after-shower powders can cause cancer.

In its natural state, some talc contains asbestos. “When talking about whether or not talcum powder is linked to cancer, it is important to distinguish between talc that contains asbestos and talc that is asbestos-free,” it says.

“Talc that has asbestos is generally accepted as being able to cause cancer if it is inhaled. This type of talc is not used in modern consumer products. The evidence about asbestos-free talc, which is still widely used, is less clear.”

The Food and Drug Administration says it has not found asbestos in any of the talcum products it has checked out. “All talcum products used in homes in the United States have been asbestos-free since the 1970s,” the American Cancer Society says.

The International Agency for Research on Cancer, the cancer arm of the World Health Organization, says genital use of talc-based body powder is “possibly carcinogenic to humans.”

But it’s difficult to go from a theory that something could cause cancer to saying it definitely caused cancer in an individual.

“For any individual woman, if there is an increased risk, the overall increase is likely to very be small,” the American Cancer Society says.

There is similar uncertainty about glyphosate. WHO's cancer agency classified it as a probable human carcinogen in 2015, but the European Food Safety Authority says it probably doesn’t cause cancer in people, and the Environmental Protection Agency has said for years that glyphosate is safe for people when used in accordance with label directions.

Cancer is very common. It is the second-leading cause of death in the United States and will be diagnosed in 1.7 million people in 2018 and kill more than 600,000, according to the American Cancer Society.

While 40 percent of cases are caused by known, preventable factors, the rest remain mysterious, frustrating scientists.

Some studies indicate that random genetic mutations cause many cases, and no single factor may be responsible.

“Cancer is a very complex process. Many abnormal cells end up going through a natural death in our bodies and are just cleared by our bodies. But sometimes those abnormal cells escape that system or something goes awry in that system, and the cells are allowed to grow,” Gapstur said.

Some judges have expressed skepticism about the lawsuits. In February, a federal judge in Sacramento blocked California from requiring that Roundup carry a label warning that it is known to cause cancer, because almost all regulators have concluded that there is no evidence that it does.

And Judge Vince Chhabria of U.S. District Court in San Francisco, who ruled that plaintiffs could present evidence about cancer in their Monsanto lawsuit, said he thought that the evidence was “rather weak” and that it would be a “daunting challenge” to persuade him to let a jury consider whether the weed killer caused any individual’s cancer.