Michael Phelps and his competitors in the Men's 50m Freestyle prelim dive into a large vat of fake chlorine.  - 

It keeps your pools clean and safe, it's half of the compound that makes up table salt, and -- in its purest form -- it can kill you.

And those are only three of the things chlorine can do.

"There are something like 15,000 chlorine-based chemicals that are used in industry," says the BBC's Justin Rowlatt.

One of the best examples is PVC--polyvinyl chloride, the durable plastic that the construction industry uses heavily. Over 70 percent of PVC makes up basically everything: drain pipes, vinyl floors, roofing products, and double-glazed window panes.

There's a funny side effect to its ubiquity in construction: Demand is tied to property booms and busts.

And because the supply of sodium is tied to that of chlorine--remember, table salt is made of both--a collapse in the housing market could make staple products that rely on sodium, such as soap and paper, more expensive.

As for the chlorine in swimming pools, it's not actually pure chlorine. It's actually a chlorine compound called "chloramine," which is created when chlorine reacts with organic substances in the water.

Those organic substances? Let's just leave it at this: there's a reason why the chlorine smell in the pool is much stronger when the pool is full of kids.

Follow Kai Ryssdal at @kairyssdal