Some of the most common causes of bloodshot eyes include dust or allergens, dry air, exposure to the sun, bacteria or viral infection, and coughing, according to Healthline. Bloodshot eyes occur when the blood vessels of the eye are irritated or swollen.
Coughing can lead to bloodshot eyes in a condition called subconjunctival hemorrhage, where a blood blotch appears on the surface of the eye. It is usually not a serious condition and resolves within 10 days, explains Healthline. Other possible reasons for bloodshot eyes are eye trauma or injury, acute glaucoma and scratches on the cornea of the eye due to contact lenses or other irritants.