Lately, I've seen some misunderstanding regarding the concept of "dose." Our predecessors once said, "sola dosis facit venenum," which translates to "the dose makes the poison." That concept still holds true today. Unfortunately, the modern anti-science movement seems to have forgotten that.
Sometimes anti-vaxxers like to worry about the few mercury atoms present in some vaccines. Other times its anti-fluoride activists complain that fluoride is added to the municipal water supplies in barrels that would be dangerous to drink from. A rather famous meme summarizes this idea pretty well, but it doesn't make sense to some people.
Here's an example that will hopefully make things make more sense:
At a high enough concentration, most things are deadly. Let's look at apples.
Apples contain formaldehyde. This is used in embalming, is a known carcinogen, and is fatal at 42mg/kg levels in mice.
You eat apples every day. Some would say that is sufficient to ward away doctors.
The EPA has determined a safe level of consuming formaldehyde at 0.2mg/kg daily for humans.
No Observable Adverse Effect Level (NOAEL) is below that continual dosage. That means you won't be injured by it under worst-case conditions.
Apples contain a tiny amount of formaldehyde (about 1mg per apple).
I weigh about 77kg. That's healthy, I think. That means I could consume 15.4mg of formaldehyde daily and be expected to suffer no ill effects over my life from that chemical.
I could eat 15 apples in a given day and still suffer no adverse effects from formaldehyde in the apple. That'd give me 15mg for the 77kg I weigh, for a total of 0.19mg/kg effective dose. I could do this every day and still be okay. I might have a tummy ache from eating so many apples, but that one chemical wouldn't poison me.
Similarly, fluoride can cause harm at huge doses in a very short amount of time. It is impossible to come remotely close to that by drinking fluoridated water. You would first die from overhydration as your liver fails or as your cellular tissue ruptures from overhydration.