For thousands of years, humans lived with minimal exposure to manmade chemicals. Today, though, we are surrounded by tens of thousands of such compounds in products we use every day.

For decades, many people assumed that the pesticides put on fruits and vegetables just disappeared, especially if you rinsed them. Most individuals didn’t worry about plasticizers in intravenous tubing and food or beverage packaging. The idea that endocrine-disrupting chemicals might be absorbed into the body seemed far-fetched.

Today, however, there is general recognition that PFAS forever chemicals are building up in our bodies. In addition, microplastic particles have been found in blood, arterial plaque, kidneys, liver, and even in the brain.

Male and female reproductive tissues also harbor microplastic particles. Although there is debate about the consequences of such exposures, many scientists worry about how such compounds might affect human health.

Now, we have a new concern. For years, we have been asking what happens to vehicle tires as they wear out. There are almost 300 million cars, trucks, SUVs, buses, motorcycles, bicycles and motor homes in the U.S. All of them use tires that must be replaced on a regular basis.

Where does all that “rubber” go? There was a time when tires were actually made of natural rubber. These days, however, tires are complex composites of dozens of chemicals beyond natural and synthetic rubber.

One in particular, 6PPD, is widely used to prevent premature tire breakdown. It moves through the tire to the surface and reacts with ozone. This prevents ozone from attacking the tire polymers and causing structural cracks.

Unfortunately, the compound formed in that reaction is 6PPD-quinone (6PPD-Q). This chemical doesn’t just disappear when tires wear out. It ends up in soil, air and water, and also in us. Biological samples have demonstrated the presence of 6PPD-Q in fish, animals and humans. It has been found in blood, urine and cerebrospinal fluid.

New research has found that this agent interacts with a number of genes that affect brain activity and neuroinflammation (Open Medicine, June 24, 2026). Several of the genes that interact with 6PPD-Q are important in the development of Alzheimer’s disease.

6PPD-Q can disrupt mitochondria, the energy factories of the cell, and lead to the accumulation of alpha-synuclein, a compound associated with the development of Parkinson’s disease.

Because neurodegenerative disorders such as Alzheimer’s and Parkinson’s diseases take many years to develop, it may be difficult to prove that 6PPD-Q is actually contributing to neurological impairment. The fact that the study identified numerous potential pathways should spur other scientists to investigate this potential risk factor for cognitive dysfunction.

One of the biggest challenges in such research is that investigating a single compound will not tell the whole story. Our bodies are the sites of countless interactions between chemicals, and we know very little about the outcomes of most of these combinations.

In addition to environmental pollutants, we are exposed to medications, cosmetics, household products and food containing a range of additives. How they affect our long-term health has yet to be determined.

In their column, Joe and Teresa Graedon answer letters from readers. Write to them in care of King Features, 300 W. 57th Street, 41st Floor, New York, NY 10019, or email them via their website: www.PeoplesPharmacy.com. Their newest book is “Top Screwups Doctors Make and How to Avoid Them.”