Atmosphere

The atmosphere of Mars is considered close to vacuum, with atmospheric pressures close to 0.6% to that of Earths mean sea level. It is composed mostly of carbon dioxide.

Temperatures of Mars range between 20 degrees Celsius at the equator, and may hit as low as -153 degrees Celsius at the poles.

The atmosphere of Mars is deadly to unprotected humans, and a space suit is required to walk the surface.

Solar Radiation

Solar radiation is a real threat on the surface of Mars. Most space suits can protect a human from the solar radiation during short exposures. The space suits however fail when they are exposed to the suns radiation for longer periods (a week). After which a person will take radiation damage on Mars' surface.

The mars stations are shielded from the solar radiation.

Coronal Mass Ejection

All Martian cities and stations are equipped with a warning system in case a coronal mass ejection is detected. Since Mars has no magnetic field to shield from such events, the inhabitants of Mars are at a great risk of dying from radiation poisoning due to coronal mass ejections.

Older stations are not properly shielded, and their citizens have to retreat to lower levels of the station to avoid being damaged by the CME. This includes the cities Phobos, Yinghuo and Deimos. The other cities have newer shielding which protects completely from most CMEs.

There are usually days between the warning of a potential CME reaching Mars, and the particles of the CME arriving at Mars.

CMEs happen roughly once a week, with some periods of turbulence of the sun causing several CMEs per day.

Any person caught on the surface during a CME will die of radiation poisoning.

Gravity

The low gravity of Mars (roughly 0.4 Earth g) causes health problems in its citizens. Although strict mandatory training regimes, and advances in research have mostly negated those health problems. Still Martians, and those visiting for longer periods, have to maintain a training regime on Mars to retain their muscle and bone mass.

Many space born settle on Mars because the intermediate gravity makes it easier for them to adapt to their new home.