There's no such thing as an all good or an all bad chemical. It all depends on it's intended purpose, the dose, and when, where, and how it's applied.
I work in a water plant and am familiar with the chemicals used, the risks of the chemical in both it's industrial concentrations and as used for water treatment, and the benefits that are gained from it's use. Let me give you a couple examples.
The debate over DMHO is one of my favorites, and I'll leave that to some of the earlier links provided.
Fluoride: we did add this for a while. The full strength chemical came in the form of Hydrofluorosylic Acid (HFSA) that was some seriously nasty stuff. It would eat through just about anything except plastic and rubber. We added it for dental health in miniscule amounts. It took us years to empty a 5000 gallon tank and we discontinued it's use when we finally ran out. It was expensive, dangerous in it's pure form, and we really didn't need it to get the benefits of fluoride. The reason was that the naturally occuring fluoride in our source (lake) water was almost at the level considered optimal for dental health. Too little fluoride makes teeth softer, just the right amount strengthens teeth, and too much turns your teeth brown. That's why topical application directly to the teeth might cause problems, too hard to control dose. Use in drinking water is perfect because it uses such small amounts that the bodies natural processes use the very small amounts over long time frames to get the benefit. A water company would have to screw up the dose for a long time to start turning peoples teeth ugly. But even then, that doesn't pose an actual health hazard.
Let me feed the chemical company consiracies a little bit here with a very rough explanation of a couple chemicals they make. They can take what is basically salt water and process it to produce two chemicals that they then sell to us water companies to use. Those chemicals are sodium hydroxide (AKA caustic) and chlorine gas. And strangely enough, once we add them into the water, the tend to combine back together and form salt water once again. It may seem like a waste since we are right back where we started, but it's necessary to get the chemicals to perform certain purposes in the water treatment process.
Caustic: just like the name says, this is a very corrosive chemical even for the half or quarter strength solutions we buy the bulk chemicals at. It's not nearly as bad as HFSA but even a drop on th skin will start to burn in just a minute or so. And it's sneaky because it just feels slick like soap at first, but as it starts to eat into the skin, the pain starts. And it's hard to wash off enough to stop the damage once it starts. It's used to adjust the pH of the water. If we don't send the water to your home with the correct pH, it can have several bad effects. Just a little off will tend to leach copper out of pipes and lead out of old solder joints used to assemble the pipe. New solder is lead free for that reason. If the pH is too low, as the natural waters in our area tend to be, it will tend to corrode the pipes from the inside out. Ideally, the pH is just right and forms a tiny protective layer on the inside of the pipes to minimize corrosion so they last a long time. If the pH is too high, the protective layer will grow too much and form scale that will clog the lines instead.
Chlorine: in it's pure form it is a corrosive gas once used for chemical warfare in WWI. If inhaled, the gas mixes with the moisture in the lungs to form acid which damages them. But this chemical is probably the single greatest boon to potable water safety ever devised. It is the main disinfectant used and is one of the most effective killers of water borne pathogens. The illnesses it prevents are too numerous to go into detail, but it's the reason things like cholera are virtually unknown in most advanced countries. It takes reletively little amounts to effectively disinfect water. Even 100 times the normal dose has no short term health effects. But there are some byproducts of the disinfection process that have become more of a long term concern as science has advanced. And even though even those risks are minimal, most companies no longer use straight gas for disinfection. By combining the chlorine with other chemicals first, we can minimize those byproducts. The drawback is that the disinfection is weaker. So it's a balancing act to make sure to eliminate the short term risk of pathogens without adding too much long term risk from the disinfection byproducts. And all these alternatives add cost since chlorine gas is the cheapest.
Ferric Chloride: this is another moderately corrosive chemical even at the 40% strength we buy the chemical at. It'll eat through some metals in nothing flat. I once accidentally used a plated brass fitting because it appeared to be stainless steel at first glance. In under 5 minutes, it was leaking because the metal just dissolved. And although the chemical has some heath risks, the most immediate problem with exposure is staining your skin. It's not too bad to work with. However, this chemical is what gets the dirt and most other things out of that cloudy lake water and does a big part of the job of turning it into clean, clear, drinking water.
Polymers: if that makes you think plastic, you're somewhat correct. These chemicals are usually fairly harmless health wise but can be messy. Several years ago, a small spill of just an ounce or two turned a 10x10 ft area of concrete into an ice skating ring on one rainy night. That stuff was literally asslick as snot, and it looked like it too. The only way to clean it up was to let it dry and form a "plastic" film that could then be chipped off the concrete. These chemicals help get the really little stuff out of the water because they are really long, sticky molecules that link the little stuff together to make it big enough to be removed by other processes and chemicals.
As you can see, many chemicals can be pretty nasty in an industrial setting. But those of us who work with them are aware of the hazards and take appropriate protective action and wear the right Personal Protective Equipment. But without them, at least as far as water is concerned, they actually serve to improve your health. Used in the proper amounts in the proper way, they turn nasty, sometimes muddy lake water loaded with harmful pathogens into clear, clean, disease free drinking water. Without these chemicals, we would be living the way they did as few as 100 years ago and dealing with routine outbreaks of waterborne diseases. Assuming you could even bring yourself to drink what they considered drinkable water back then. Even now, in areas of the world where such chemical treatment is not available, high quality drinking water is in short supply.
As I read back over this, I see it's probably too complicated in some ways and oversimplified in other ways. But hopefully, it shows how "dangerous" chemicals can be used to good purpose in producing something that we all need to live.
Just because you cannot determine fact from fiction does not mean nobody else can. The general public is not as stupid as you think.
