could people live in the Precambrian Era? I mean, assuming they

could get there via time travel.

Or how about the Coal Age or the time of the dinosaurs?

Wikipedia has a graph here based on this paper: http://rstb.royalsocietypublishing.org/content/361/1470/903

Humans can't live permanently above 20,000 ft or so now, due to the lack of oxygen, which is roughly half the oxygen at sea level: "Highest human habitation at is at 6000 m and 380 mm Hg (barometric pressure)." http://hypertextbook.com/facts/2005/MoniqueAnthony.shtml

So maybe a little before the start of the Cambrian might have been possible, but not earlier.

How far back do you want to go?
Depending on the time period, the atmospheric oxygen levels may not be high enough.
You'd need some serious sunblock too, as the ozone layer hadn't formed yet either.

to human life. Plus weren't all those radioactive elements much more radioactive back then?

The Carboniferous, however, would have been almost perfect, it was an "icebox" world much like our own, with recurring cycles of ice ages and interglacial periods. The changes in sea level causes by the expansion and contraction of the Gondwana Ice Sheet is responsible for the the great coal deposits of Western Europe and the Appalachians. Swampy tropical lowlands got flooded and covered with marine sediment on a regular basis, creating layers of burred plant matter that became coal. the only fly in the ointment, though, is that oxygen levels were over 30% and even the swampy plants show adaptations for intense fires, so be careful with that match!

The Mesozoic would have been livable, but most places would have been very hot and muggy. what is now spruce forest resembled Florida or Louisiana in the Late Cretaceous.