With the unofficial start of summer on Monday, many people will get up close and personal with the element that carries 17 protons.

I speak, of course, of chlorine.

Over the next few months, chlorine will ensure that countless swimming pools don’t turn into microbe-choked petri dishes. That’s only one of many uses we’ve found for the element. We sprinkle it on our food as table salt — a k a sodium chloride. We pump water through pipes made of polyvinyl chloride. Perchlorate, a combination of chlorine and oxygen atoms, fuels rockets and ignites fireworks.

But in other incarnations, chlorine is a bane of our existence. In World War I, the German army unleashed clouds of chlorine gas and killed or injured thousands of enemy -soldiers. The Hudson River is burdened with cancer-causing dioxins, chlorine-bearing compounds dumped from factories along its banks.

Still, chlorine’s threats today are nothing compared with its menace on the early Earth.

According to a study recently published in the journal Earth and Planetary Science Letters, the entire planet was poisoned with the stuff. Fortunately, the planet got rid of most of its chlorine. If it hadn’t, we might not be here today.