Our eyes are incredibly sensitive. That comes as no surprise; they’re incredibly complex organs that allow us to understand the universe in precise detail. We intuitively appreciate the importance of our eyes, so it’s no wonder that people grow concerned when they see any signs that their vision might be off, from floaters to red eye. Fortunately, in most cases, when your eyes are red it doesn’t mean you’re in any immediate danger of lasting damage to the eye. We’ll go through a few reasons why your eyes might be red, then explain when you should visit your optometrist.

One of the leading causes of red eye is irritants. These irritants might be particles, like dust or pollen. Low quality, stale air can also be an irritant; when the air is too dry, or there’s air pollution and airborne fumes, you might experience red eye. Anyone who thought it was a good idea to open their eyes underwater in a public pool has experienced red eye; chemical irritants like chlorine will cause discomfort and redness. UV rays can also cause red eye; if you’re particularly prone, consider eyeglasses with photochromic lenses. These lenses are clear when no UV light is present, but transition to UV blocking sunglasses when exposed to the sun.

There’s a variety of eye conditions that can cause red eye. The most common are dry eye and conjunctivitis, more commonly known as pink eye. Eye allergies can cause red eye as well, mostly brought on by the irritants mentioned above. Your eyes will be red after almost any ocular surgery, including laser-eye surgery; abrasions to the eye causing corneal ulcer and any type of eye trauma can also cause red eye. It’s worth mentioning that if your eye has been injured in any way, you should go to an optometrist immediately; damage to the eye might constitute an emergency.

There are a number of routine things that can also cause red eye. Your eyes may become red if you’ve worn your contact lenses for too long. Digital eye strain can also cause redness. A variety of drugs can also cause red eye; ask embarrassed young adults what they’re doing picking up Visine and 3 bags of Doritos at the drugstore at 11 o’clock at night and you’ll understand what I mean. Smoke in particular is prone to causing red eyes, as it too is an irritant.

So when do you go see an optometrist about red eye? The answer varies, but if you don’t know what’s causing it, or it seems to be persistent, it’s probably a good idea to see a medical professional. Changes in vision accompanied by red eye are another sign that it’s time to see a doctor; while our eyes are sensitive, the redness shouldn’t be accompanied by any blurring or double vision. When you’re not sure, I always recommend erring on the side of caution and visiting the optometrist; better safe than sorry, after all!