These are called subconjunctival hemorrhages, and are harmless to the eye and do not hurt the vision. They can result from injury, high blood
pressure, taking blood
thinners, straining, coughing, sneezing, contact lens wear. Despite their appearance, they are not dangerous. There is no way to prevent them. If you have recurrent hemorrhages, I would have a work-up with your medical doctor to be sure you do not have a clotting disorder, but this is very rare.