What killed the dinosaurs? Ok, ok, we know an asteroid did it, roughly, but that made a comparatively small hole in the ground -- what actually killed them? We've heard lots about nuclear winter, global wildfires, all sorts of poisonous gases, and combinations thereof. But what if it was something far more mundane. What if it was ozone?

At the start, it seems hard to make a potent mass murderer out of a gas that is just three little oxygen atoms bounds together. And we all know about the ultraviolet-shielding effect of the ozone layer high in the stratosphere. Very helpful for life, that is.

But problems start when you bring O3 down near the surface. Mix together a soup of hydrocarbons, nitrogen oxides, and sunlight (the first two make up your basic car exhaust) and presto, we can find ourselves swimming in the stuff, especially in urban areas. No one knows what safe levels are, but medical studies suggest that lung tissue gets inflamed and damaged quickly at or around 100 parts per billion of O3.

A new study in the journal Paleogeography, Paleoclimatology, Paleoecology puts forth the idea that the Chicxulub impact, long blamed for the extinction of the dinosaurs at the end of the Cretaceous era 65 million years ago, could have done them in by flinging huge amounts of ozone precursor chemicals -- nitrogen oxides, methane, and other hydrocarbons -- into the air.

According to the researchers' simulation, the impact could have produced enough ozone to raise concentrations in the atmosphere to over 1,000 parts per billion (or 1 part per million), about 10 times the dangerous level for people).