People who are exposed to certain forever chemicals may be at greater risk of developing multiple sclerosis (MS), according to new research.

No one knows why that is, but it could help explain why, over the past 30 years, the prevalence of MS has increased by an average of 26 percent globally. In some nations, cases have more than doubled since 1990.

MS is an autoimmune disease of the central nervous system with no known singular cause and no known cure.

This new study from Sweden suggests that forever chemicals may be an overlooked contributing factor.

Related: Multiple Sclerosis May Have Two Distinct Subtypes, Scientists Discover

Over the decades, studies have linked the risk of MS to several genetic variations and to some key environmental exposures, such as the Epstein-Barr virus.

Some research has considered the potential role of forever chemicals – more formally known as per- and polyfluoroalkyl substances (PFAS).

How we are exposed to forever chemicals. (European Parliamentary Research Service/Flickr)

Using Swedish health data, researchers measured 24 PFAS compounds in the blood of 907 patients recently diagnosed with MS and 907 healthy controls. They also considered seven byproducts from other forever chemicals, called polychlorinated biphenyls (PCBs).

In the end, participants whose blood contained a higher concentration of these chemicals had a significantly higher risk of MS.

Some of the strongest associations occurred from a combination of PFAS compounds and/or their byproducts, rather than a single chemical on its own.

The ‘toxic synergy’ of forever chemicals is a complication that scientists have warned about before.

"The results show that when attempting to understand the effects of PFAS and other chemicals on human beings, we need to take mixtures of chemicals into account, not just individual substances, as people are generally exposed to several substances at the same time," says first author and medical researcher Aina Vaivade from Uppsala University in Sweden.

Since the mid-twentieth century, forever chemicals like PFAS have been used in a wide range of products, including non-stick pans, stain-resistant fabrics, fire-fighting foams, and cosmetics galore.

Today, forever chemicals are virtually everywhere: in water, beverages, food, our guts, our blood, our brains; they can even seep through our skin via cosmetic products.

To make matters worse, studies are increasingly turning up adverse health connections associated with certain chemicals at certain concentrations.

Of the more than 12,000 PFAS chemicals produced, only two, PFOA and PFOS, show clear links to cancer and birth defects. While both are now phased out of production in many nations like the US, they take ages to break down naturally in the environment, making them persistent threats even to this day.

In the current research, participants with either higher levels of PFOS or one of two PCB byproducts (4-OH-CB187 and 3-OH-CB153) showed especially strong odds of developing MS.

"We saw that several individual substances, such as PFOS and two hydroxylated PCBs, were linked to an increased odds for MS," explains lead author and clinical chemist Kim Kultima from Uppsala.

"People with the highest concentrations of PFOS and PCBs had approximately twice as high odds of being diagnosed with MS, compared with those with the lowest concentrations."

PFOS and OH-PCBs can both cross the blood-brain barrier, possibly infiltrating immune cells in the central nervous system.

If these chemicals induce oxidative stress, researchers suspect they could impair antioxidant defenses in the brain, possibly contributing to the muscle weakness, numbness, or vision impairment often associated with MS.

Strangely enough, Kultima and colleagues found that the odds of developing MS in patients with a gene variant tied to a lower risk of the disease were instead much higher if they had been exposed to greater levels of PFOS.

In fact, with increasing PFOS exposure, participants with this gene variant had a greater than fourfold higher risk of developing MS.

"This indicates that there is a complex interaction between inheritance and environmental exposure linked to the odds of MS," explains Kultima.

"We therefore think it is important to understand how environmental contaminants interact with hereditary factors, as this can provide new knowledge about the genesis of MS and could also be relevant for other diseases."

The study was published in Environment International.