Yeah, +20 C is fine! One of few things being fine here unfortunately!
The surface pressure on Mars is around 600 Pa, compared to over 100,000 Pa on Earth. The minimum a human can withstand is around 6300 Pa (the [Armstrong limit](https://en.wikipedia.org/wiki/Armstrong_limit
)). So while you tell you are using an oxygen mask, a bit of an issue here is that eventually your blood will start to boil. 6300 Pa gives a boiling point of water/blood at +37 C / +100 F, the normal body temperature.
Mars lacks a thick atmosphere and a global magnetic field, so the radiation that reaches the surface from the Sun and space is much different than on Earth and more like how it is on the Moon.
Thanks to the Curiosity rover, we now know that the dose for a 180 day journey is 300 mSv, which is about 24 CAT scans. An explorer going to Mars would be exposed to >15x the annual radiation limit for nuclear power plant workers.
Here's what NASA has to say about this:
> For a six-month journey to Mars an astronaut would be exposed to roughly 300 mSv, or a total of 600 mSv for the round-trip.
If we assume that the crew would spend 18 months on the surface while they wait for the planets to realign to make the journey
back to Earth possible, they will be exposed to an additional 400 mSv, for a grand total exposure of about 1,000 mSv. Note that
an astronaut repeating the same journey on multiple occasions could receive less or more radiation each time, depending if they
are in the line of a CME or SPE.
Having said that, yeah, I guess a human would likely survive it, if wearing a pressure suit. Yeah, that full 1000 mSv fancy irradiated Mars vacation round trip estimate that NASA gave. A total of 1000 mSv bombarding you in a _single day_ would put you in immediate danger with moderate nausea and headache (acute radiation poisoning), with a 5% risk of death in one month, otherwise non-fatal ([source](http://radiationsurvival.blogspot.com/
)). And this wouldn't be during a single day, but stretched out over 18 months. (unsure how/if this changes the severity of the exposure?) But it would obviously drastically increase the risk of cancer in case you arrive back on Earth alive and all.
> Approximately 134 plant workers and firefighters battling the fire at the Chernobyl power plant received high radiation doses – 800 to 16,000 mSv – and suffered from acute radiation sickness. Of these, 28 died within the first three months from their radiation injuries. Two more patients died during the first days as a result of combined injuries from the fire and radiation.
(same source as above)
While radiation is nasty, many articles and discussions on this seem to overestimate the radiation problem. One article I came across said it would plain kill you right out. This is not an acute problem on Mars from what I can tell. In that case, the pressure is a hell of a lot more. While you're worrying if you'll die from cancer back on Earth, your body will boil! Having said that, the radiation problem becomes _very real_ over time, and the problem is that "over time" is exactly what we _always_ end up with due to planetary alignment planning for trips there and back, making missions easily span a year. _Then_ radiation is a problem.
Unfortunately, the Martian atmosphere is extremely thin - about 0.5% of the pressure of the atmosphere on the Earth's surface. On a human scale, this would feel basically like a vacuum, and you would have all of the issues that come with vacuum exposure. For example, in the 1960s a NASA test subject was accidentally exposed to a vacuum similar to the pressure of the Martian surface, and was unconscious within seconds.
You would still need a pressurised space-suit, it just wouldn't need to have much heating in it.