The Risks of a Manned Mission to Mars, Part V

Another potential hazard on the surface of Mars is the dust storms. They can be planet-wide, and can also last for months. These dust storms make it difficult for the astronauts to go outside and collect data. A study, led by University of California physicist Gregory Delory, found that “Martian dust storms can generate electric fields that rip apart water and carbon dioxide molecules in the planet's thin atmosphere. Those molecules would then lead to the formation of hydrogen peroxide and other corrosive chemicals that fall to the Martian surface as a sort of toxic snow.” These toxic chemicals could potentially put the astronauts at great risk.

http://www.space.com/scienceastronomy/060731_mars_duststorms.html