The Risks of a Manned Mission to Mars, Part I

One of the biggest health-related risks involved with interplanetary spaceflight is the long-term exposure to solar and cosmic radiation. Once outside the protection of Earth’s magnetic field, the astronauts will be exposed to two sources of radiation: solar particle events (SPE) and galactic cosmic rays (GCR). SPE are high-energy particles emitted by the Sun during intense flares, while GCR are particles that originate outside the solar system. These radiations can increase the risk of cancer and damage the reproductive systems of the astronauts. Including the six months of travel time each way, a 2 ½ year round trip to Mars would expose the astronauts to nearly the lifetime limit of radiation allowed under NASA guidelines.

http://aerospacescholars.jsc.nasa.gov/HAS/cirr/em/9/4.cfm