Dryness, excessive sunlight, allergies, coughing, dust and infection are common causes for bloodshot eyes. These irritants induce swelling of the vessels in the white portion of the eyes.
Eye infections or inflammation can cause itchiness, pain and vision problems. These include blepharitis, conjunctivitis, uveitis and corneal ulcers. Straining and coughing can sometimes result in a dense bloody spot in the eye called a subconjunctival hemorrhage. This is common and usually clears up within a couple of weeks. Another cause of bloodshot eyes is acute glaucoma, which is a sudden increase in pressure. Acute glaucoma is typically chronic and gradual and is a medical emergency.