The other answers here talk about the hazards of the perchlorates, carbon dioxide etc. Though those are of course important if we do ever have humans on Mars, for me the main thing is planetary protection. Until they show that it is possible to send humans to the Mars surface without contaminating it irreversibly with Earth life, then they shouldn't land humans on Mars.
Instead, study it from orbit, or land humans on Deimos or Phobos.
Why send humans to the surface? And risk turning what is potentially the biggest discovery in the recent history of biology into an "Oh shoot, we've contaminated this planet with Earth life, this is just a microbe which arrived there in the crash landing of that human colonist attempt ten years back".
That would be one of the biggest anti-climaxes in science in recent past. We simply must not let this happen.
It can be studied from orbit using telepresence, which actually gives you a more immersive experience than if you are on the surface yourself. Digitally enhanced streaming HD stereoscopic video from the surface.
On the surface, you'd be in a clumsy spacesuit which takes maybe an hour a day to put on and take off, need to check everything thoroughly as your life depends on it - and eyes poorly adapted to the dim yellowish brown light levels there. You can't distinguish colour easily (the photographs you see from Mars are all digitally enhanced for human eyes) - and you would not be permitted anywhere near anywhere on the surface that is at all likely to have present day life.
Instead their plan is to land humans somewhere on Mars far enough from anywhere biologically interesting not to contaminate it. And then send robotic explorers to pick up biologically interesting samples to return to the human settlement. But in that case - why have humans on the surface in the first place? And how can you keep human contamination confined to the human base, when you have global dust storms, and the dust particles are of iron oxide which can shield microbial spores from UV radiation and transfer them to anywhere on the planet world wide?
And what if a human mission crash lands? 50% of all missions to the Mars surface have failed. That's got better with advances in technology but a human mission there is such a huge step up in technology that it surely is bound to have significant risk of a crash landing.
Anyway, spacesuits leak, and habitats are not likely to be 100% closed system but would vent waste gases and almost certainly some human wastes as well, and each time you use the airlock, countless microbes drift out onto the surface of Mars. Many would die but some would fall into a shadow, protected from UV light, get into a dust grain, and if it is a microbial spore, it is now a go to get transferred in the winds anywhere on the surface of Mars, and maybe eventually drift into a habitat if such exist there.
There are many other suggestions to explore Mars from orbit. Or from Deimos or Phobos - this is one artist's impression, made for Lockheed Martin, of a mission exploring Mars from Deimos.
There's also the HERRO idea of exploring it from a Molniya orbit that approaches the sunny side of Mars twice every Mars day so you get to see the entire planet close up in full sunlight each Mars day, a spectacular orbit.
And from orbit, then you have direct access to the entire surface of the planet, or 90% of it or so, depending on orbit and where you are. And can control telerobots on the surface anywhere on Mars.
So one crew member in orbit can do as much as several humans on the surface. And many technologies such as rovers capable of supporting and transporting humans, and systems to safely land humans on Mars and take off again simply not needed. Those are technologies likely to be at least as tricky to develop as the Space Shuttle, just not needed if you study it from orbit.
I think that we should study Mars either from Earth, as we are now, or if funding permits, with humans in orbit around Mars. But shouldn't send humans anywhere near the surface until we begin to have a reasonably thorough understanding of what is there and what the impact of humans would be.
And that may take a fair while, some decades, it is a large and complex planet, same surface area as the land mass of the Earth and microbial life would be invisible from orbit even if on the surface or near sub surface and likely to be in low concentrations and extremely long lifetimes, if you use the analogy of similar habitats on Earth - no detectable effect on the atmosphere, and individual microbes or lichens etc could have lifetimes of thousands of years slowly metabolizing.
Those who say we should go to the surface tend to say we must do it because it is more interesting and exciting, and having gone so far surely we should go the last little bit down to the surface.
But - I'm sure the general public can come to understand why we are exploring it from orbit rather than the surface. That actually adds to the interest I think rather than detract, something fascinating about a location you can't actually go to in person.
And we've seen how much interest there is in robotic exploration with New Horizons, Dawn, Curiosity. I think in some ways for the general public, not space geeks, it overshadows news from the ISS. I think there'd be a lot of interest in science exploration of Mars from orbit around it, and any science discoveries, of life there etc, would be big news, and everyone would understand, once explained, why you study it from orbit with only robots in situ.
In any case, at this stage, we simply shouldn't send humans to the surface if it is going to greatly increase risk of irreversibly introducing life to Mars. And we don't know enough yet, and I don't think we will know for some time, what the biological effect on Mars would be of a human mission and especially a crash landing on the surface.
See also
To Explore Mars With Likes Of Occulus Rift & Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos
which goes into more detail of various orbital proposed missions from Lockheed Martin, Russian space agency, NASA etc.
And several other articles I've written on these topics on my Science20 blog.
For possible locations for life habitats on Mars surface, see
Habitability Of Mars - Salty Seeps, Liquid Layers In Polar Ice, Ice Fumaroles, Sand Dune Bioreactors. ...
Or less technical:
How Life May Exist On Mars With Atmosphere Close To Equilibrium
Where To Search On Mars For Droplets, & Shallow Flows Of Liquid Water - Where Microbial Life May Flourish
Our Spacecraft Could Look Straight At an Extraterrestrial Microbe - And Not See a Thing!
Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know
Might there be Microbes on the Surface of Mars?
Instead, study it from orbit, or land humans on Deimos or Phobos.
Why send humans to the surface? And risk turning what is potentially the biggest discovery in the recent history of biology into an "Oh shoot, we've contaminated this planet with Earth life, this is just a microbe which arrived there in the crash landing of that human colonist attempt ten years back".
That would be one of the biggest anti-climaxes in science in recent past. We simply must not let this happen.
It can be studied from orbit using telepresence, which actually gives you a more immersive experience than if you are on the surface yourself. Digitally enhanced streaming HD stereoscopic video from the surface.
On the surface, you'd be in a clumsy spacesuit which takes maybe an hour a day to put on and take off, need to check everything thoroughly as your life depends on it - and eyes poorly adapted to the dim yellowish brown light levels there. You can't distinguish colour easily (the photographs you see from Mars are all digitally enhanced for human eyes) - and you would not be permitted anywhere near anywhere on the surface that is at all likely to have present day life.
Instead their plan is to land humans somewhere on Mars far enough from anywhere biologically interesting not to contaminate it. And then send robotic explorers to pick up biologically interesting samples to return to the human settlement. But in that case - why have humans on the surface in the first place? And how can you keep human contamination confined to the human base, when you have global dust storms, and the dust particles are of iron oxide which can shield microbial spores from UV radiation and transfer them to anywhere on the planet world wide?
And what if a human mission crash lands? 50% of all missions to the Mars surface have failed. That's got better with advances in technology but a human mission there is such a huge step up in technology that it surely is bound to have significant risk of a crash landing.
Anyway, spacesuits leak, and habitats are not likely to be 100% closed system but would vent waste gases and almost certainly some human wastes as well, and each time you use the airlock, countless microbes drift out onto the surface of Mars. Many would die but some would fall into a shadow, protected from UV light, get into a dust grain, and if it is a microbial spore, it is now a go to get transferred in the winds anywhere on the surface of Mars, and maybe eventually drift into a habitat if such exist there.
There are many other suggestions to explore Mars from orbit. Or from Deimos or Phobos - this is one artist's impression, made for Lockheed Martin, of a mission exploring Mars from Deimos.
There's also the HERRO idea of exploring it from a Molniya orbit that approaches the sunny side of Mars twice every Mars day so you get to see the entire planet close up in full sunlight each Mars day, a spectacular orbit.
And from orbit, then you have direct access to the entire surface of the planet, or 90% of it or so, depending on orbit and where you are. And can control telerobots on the surface anywhere on Mars.
So one crew member in orbit can do as much as several humans on the surface. And many technologies such as rovers capable of supporting and transporting humans, and systems to safely land humans on Mars and take off again simply not needed. Those are technologies likely to be at least as tricky to develop as the Space Shuttle, just not needed if you study it from orbit.
I think that we should study Mars either from Earth, as we are now, or if funding permits, with humans in orbit around Mars. But shouldn't send humans anywhere near the surface until we begin to have a reasonably thorough understanding of what is there and what the impact of humans would be.
And that may take a fair while, some decades, it is a large and complex planet, same surface area as the land mass of the Earth and microbial life would be invisible from orbit even if on the surface or near sub surface and likely to be in low concentrations and extremely long lifetimes, if you use the analogy of similar habitats on Earth - no detectable effect on the atmosphere, and individual microbes or lichens etc could have lifetimes of thousands of years slowly metabolizing.
Those who say we should go to the surface tend to say we must do it because it is more interesting and exciting, and having gone so far surely we should go the last little bit down to the surface.
But - I'm sure the general public can come to understand why we are exploring it from orbit rather than the surface. That actually adds to the interest I think rather than detract, something fascinating about a location you can't actually go to in person.
And we've seen how much interest there is in robotic exploration with New Horizons, Dawn, Curiosity. I think in some ways for the general public, not space geeks, it overshadows news from the ISS. I think there'd be a lot of interest in science exploration of Mars from orbit around it, and any science discoveries, of life there etc, would be big news, and everyone would understand, once explained, why you study it from orbit with only robots in situ.
In any case, at this stage, we simply shouldn't send humans to the surface if it is going to greatly increase risk of irreversibly introducing life to Mars. And we don't know enough yet, and I don't think we will know for some time, what the biological effect on Mars would be of a human mission and especially a crash landing on the surface.
See also
To Explore Mars With Likes Of Occulus Rift & Virtuix Omni - From Mars Capture Orbit, Phobos Or Deimos
which goes into more detail of various orbital proposed missions from Lockheed Martin, Russian space agency, NASA etc.
And several other articles I've written on these topics on my Science20 blog.
For possible locations for life habitats on Mars surface, see
Habitability Of Mars - Salty Seeps, Liquid Layers In Polar Ice, Ice Fumaroles, Sand Dune Bioreactors. ...
Or less technical:
How Life May Exist On Mars With Atmosphere Close To Equilibrium
Where To Search On Mars For Droplets, & Shallow Flows Of Liquid Water - Where Microbial Life May Flourish
Our Spacecraft Could Look Straight At an Extraterrestrial Microbe - And Not See a Thing!
Rhythms From Martian Sands - What Did Our Viking Landers Find in 1976? Astonishingly, We Don't Know
Might there be Microbes on the Surface of Mars?
About the Author
Robert Walker
Writer of articles on Mars and Space issues - Software Developer of Tune Smithy, Bounce Metronome etc.Studied at Wolfson College, Oxford
Lives in Isle of Mull
4.8m answer views110.3k this month
Top Writer2017, 2016, and 2015
Published WriterHuffPost, Slate, and 4 moreHuffPost, Slate, Forbes, Newsweek, Business Insider, and Science 2.0