A foreign body presents inside the eye (dust or sand) can cause abrasions to the eye's surface. Corneal abrasions are very uncomfortable and cause eye redness, eye pain and severe sensitivity to light. In the presence of a foreign body and/or corneal abrasions, the eye becomes more susceptible to infection. Don’t try to remove the foreign body yourself as you may cause even more injury to your eye. If it’s very painful to keep your eye open, simply close your eye and see your doctor as soon as possible.
Having splashed or sprayed unknown substances in the eye can be dangerous.
Acid can cause considerable redness and burning. It can be washed out fairly easily.
Alkali (toilet bowl cleaners, oven cleaner) don't cause as much immediate eye pain or redness as acids however they are much more dangerous.
If you have unknown substance in the eye, wash your eyes immediately with water for a minimum of 10 minutes. Then call and visit your eye doctor as soon as possible.