What kinds of pollutants come from an airplane and what harm do they do to the environment. Also,what are the dangers and/or hazards of airplanes?

The most important pollutants which come from airplanes are more or less the same as those from cars and power stations. They are carbon dioxide (CO2) and nitrogen oxides (NO2 and NO, often called NOx). Burning of hydrocarbon fuels (aviation fuel) also produces a lot of water vapour, itself a strong greenhouse gas, which forms condensation trails. These con trails increase cloudiness which tends to trap warmth below them and stop it from escaping by radiation back into space, just like CO2 gas. Con trails are one reason why aircraft emissions are more damaging at the high altitudes at which they fly (8-13 km). CO2 and water vapour are the main global warming gases from jet exhausts but there's a rather complicated reaction between the NOx gases, sunlight and oxygen which creates ozone (O3) gas which is itself a greenhouse gas. Aircraft also emit unburnt hydrocarbons, soot, and sulphate particles. You may want to look at my global warming guide.
   
Apart from the global warming gases I mentioned, NOx adds to acid rain which harms lakes and forests in many parts of the world.
    The dangers of airplanes are mainly that there are so many flying around all the time that they are doing a lot of damage to the atmosphere. Other dangers are obvious: they can crash for various reasons. Terrorists have also discovered that they can be used as missiles if they are hijacked. They can also be used by military forces for dropping bombs on cities and killing lots of people.