So often stress is considered an amorphous gray area—something we can’t put our finger on or measure that gets dismissed as not being “real.” But I believe that what we think and feel, and how long we think it or feel it, determines our health. Here are 10 concrete ways stress is possibly the most dangerous toxin your body faces every day.

The chemicals your body produces when you are under stress turn on or off of genes that change everything from how much fat you store, to how well your immune system works, to how fast you age, to whether or not you will develop cancer.

Research shows that even very early childhood events “set” your CRH, or corticotropin releasing hormone, at a high or low level. CRH is like the foot on the gas turning on your adrenals, and therefore your stress levels.

High levels of stress hormones damage critical parts of the brain, such as the hippocampus, the area responsible for memory. One reason people experience “adrenal burnout” after long term chronic stress, is because the brain, in order to save itself, turns off the adrenals.

From slowing wound healing to diminishing the protective effects of vaccines to increasing your susceptibility to infections, stress is the ultimate immune-modulator. Stress can also reactivate latent infections—people who get cold sores know this from experience.

These energy factories produce ATP, the currency through which all cells and organs in your body do their work. The good news is this damage is reversible over time, as stress goes away.

Studies have shown that the activity of hundreds of genes responsible for enzymes that break down fats and detoxify prescription drugs, are negatively impacted by stress. Stress can also increase your toxin burden by increasing your desire for high-fat, high-sugar foods.

Chronic stress has been shown to increase the thickness of the artery walls, which could lead to high blood pressure and heart disease.

Stress increases the amount of something called sex hormone binding globulin, the school bus that ferries testosterone and estrogen around your body, meaning fewer of these hormones are available to your cells. Chronic stress also increases the production of cortisol, leading to something called “cortisol steal,” where fewer sex hormones are produced.

There is evidence that higher stress levels are associated with lower bone mineral density, and studies show that people under chronic stress experience more physical pain.

You may have heard that 95% of your serotonin is in your gut, and you may remember a time when you were nervous or sad, and your belly was in knots. But more research is showing how stress impacts the function of your gut every day. It slows transit, leading to constipation and the re-circulation of hormones like estrogen through your liver. It increases the overgrowth of bad bacteria. And it loosens the barriers between the cells that line the intestines, creating something called leaky gut, that then leads to inflammation, food sensitivities and even autoimmune disease.