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.