If / when humans finally reach another planet that has life, what are the main challenges facing humans living in the open trying to colonise a planet with an alien biology?
Assume that the planet closely resembles the earth in all respects except that its evolution has run a completely different course due to random variation. Plants and animals exist and oxygen and carbon dioxide are consumed and released as they are on earth, but all metabolic processes are carried out by an entirely different set of organic chemicals. No DNA and no proteins exist. This different set of organic chemicals range from the harmless to the toxic and everything in-between. All life forms are inedible and most are poisonous to varying degrees. There are no sentient inhabitants.
edit Assume that the environment (plants animals and soils) are mildly toxic to all earth based life. There are some non-toxic (but inedible) species and a few very toxic species. Think of the environment containing many of the less toxic reagents from an organic chemistry lab.
Assume that most fine particulate matter such as dust and the equivalent of pollen are either non-toxic or easily filtered out by a simple face mask.