Mars is a cold planet filled with deserts, and lacking oxygen or gravity. The average annual temperature on Mars is -64°F. At its hottest, our next door neighbor could hit 80°F near the equator in the middle of the day. In Martian winter, temperatures could fall to -199°F.

Obviously for anyone who were to visit Mars, a space suit would be an absolute requirement. But how long could someone survive a summer day on the Red Planet without wearing protective equipment? What if that someone wore only jeans and a t-shirt and held their breath for a minute?

Chris Webster from NASA's Jet Propulsion Laboratory (which manages the Curiosity rover currently roaming on Mars, said in an email:
The most serious immediate impact would be from the low atmospheric pressure that is nearly a vacuum compared to Earth, and within minutes the skin and organs would rupture, outgas and produce a quick painful death.
And then there are other environmental factors that make Mars a hostile ground for humans without protection.
Any humans on Mars would have to contend with the lack of oxygen (only ~0.1% compared to Earth’s 20%), the very cold surface temperatures, the ubiquitous and irritating dust, the intense UV radiation, surface chemicals and oxidants — and all this before they started looking for food and water!
That sounds awful. We're very happy to remain on earth and look at Mars through Curiosity rovers, thank you very much.