Why are heavy metals dangerous?
The short answer is that heavy metals react with your cellular machinery in ways that are often problematic. Small amounts can get filtered out with your urine, but heavy metals mostly stick around in various tissues, as well as in your bones and blood, so taking in too much can cause the metals to accumulate—that’s when you run into problems.
For some, reaching a dangerous level is tough. Heavy metals like iron and zinc, are not just useful but essential for your body to function. Iron allows your red blood cells to bind oxygen molecules. Various enzymes in your body require zinc in order to work properly. Too much of either would result in similar symptoms to other heavy metal poisonings, we just don’t commonly get exposed to dangerous levels so you don’t hear about them when we talk about heavy metal poisoning.
Other metals, like copper, are both necessary and alarming. You need trace amounts to help enzymatic reactions inside your cells, but you can get copper poisoning if your drinking water is contaminated (the EPA considers anything over 1.3 milligrams per liter to be too much). The same reactivity that allows it to feed reactions also causes it to produce free radicals, which can damage DNA and other cellular components.
Many of the heavy metals you hear about, though, fall into a third category: safe in small quantities, but mostly just dangerous. Arsenic, for instance, is virtually everywhere, so it’d be close to impossible to avoid it entirely. But in larger than trace quantities, it’s a huge problem. It inhibits a wide variety of enzymes, and arsenic poisoning manifests as a host of upsetting symptoms including seizures, brain hemorrhages, vomiting, bloody diarrhea, anemia, muscle aches, and hardening of the skin.
Not all heavy metals give you quite this range of dire medical manifestations, but most of them will give you severe gastrointestinal and nervous systems problems. We’re not even sure precisely how most of the heavy metals damage the human body, but the common thread is that they interrupt enzymes and interact with DNA in such a way that basic cellular processes shut down.
Lead is the main exception to the rule, it primarily exerts its influence on the human body thanks to its molecular mimicry of calcium (though it also interferes with enzymes.) Your nerves rely on calcium to send signals to each other. All the protein channels and enzymes that use calcium to open or close get confused by lead, prompting them to malfunction as the metal accumulates. Researchers aren’t sure of all the mechanisms by which this interference causes problems, but the end result is a variety of neurological issues. Symptoms can take time to show up, but chronic ingestion causes confusion, headaches, seizures, and in the long-term prevents neurological development, making it particularly worrying for young children whose brains and bodies are still developing.