As I am doing research for a new short story my mind has been working on the concept of humans living on an alien planet and that brings up the difference in my mind between inhospitable environments contrasted against lethal ones. My working definition of a lethal environment is one where absent direct technological intervention will kill a person outright; vacuum, poisonous atmospheres, and the like are examples of such environments while an inhospitable environment may be deadly it is not so immediately. A desert here on Earth is very inhospitable and if I dropped you in the middle of one you may last days before you die from exposure and if you were very lucky very smart or both, you may be able to find the essential resources to survive. Clearly an alien desert would be inhospitable for the exact same reasons as an earthly one, but I think that analysis is faulty and in that it is too generous. I think that all alien ecosystems with be inhospitable to terrestrial life.
Often alien planets are presented as Earth analogs, the exact nature and shape of the animals and plants may vary in interesting ways but essential a person walking their lands is exploring fun house version of out home. This ignores a number of very basic biological factors.
Consider Allergies.
Your immune system when it encounters a foreign substance in your tissues classifies it as either invader or something to be ignored. (Clearly it is a lot more complicated than that but the simplification works for this exercise.) Harmful bacteria, viruses and the like provoke and immune response to defend out health but sometimes the immune system identifies something that should be ignored as an invader and it reacts when it should not. For commonly encountered substances, such as dander and pollen, when this happens we call that reaction an allergy. Even if you suffer from common allergies your immune system ignores quite a lot of material floating in the wind. It encounters this material, recognizes it and ignores it. However on an alien would all that material will be substances that your system is not keyed to ignore and it is very likely that humans, and any animal we bring with us, may suffer allergic reactions to nearly everything in an alien ecosystem. At least one Apollo astronaut is thought have had an allergic reaction to lunar dust.
Eating will be out of the Question.
In the episode of Star Trekthe original series The Way to Eden(S3E20) when the flower-children analogs reach Eden and eat of the fruit they find that is poisonous. The truth of the matter is that alien plants and animal are, at best, gong to be indigestible to Earth life. Digestion is a complex series of chemical reactions facilitated by very specific enzymes. A simple and well-known example is lactase the enzyme that allows people to digest the lactose found in dairy, once a person stops producing lactase they can no longer digest milk products. Think about all the things found on Earth that you cannot eat. By far there are more inedible substances in nature than edible ones and that is in an ecology that humans have evolved within. Without the specific enzymes required plants and animals from an alien ecology will be inedible.
A human stranded on an alien world will suffer massive allergic reactions and slowly starve to death.
