No one likes looking in the mirror to find red eyes, looking back. Typically, it’s nothing to worry about — often just a harmless reaction to dust or allergens. But that’s not always the case. There are times when inflamed eyes are a signal that you need a doctor’s attention right away.

How can you tell the difference between common, benign reasons for irritation and more unusual, concerning problems?

“There are really three important symptoms that should make someone seek attention if their eyes are bothering them,” says ophthalmologist Richard Gans, MD. “If, besides redness, there is  pain, extra light sensitivity or blurred vision, patients should get it checked out.”

He outlines five conditions that can cause people’s eyes to become irritated and red:

1. Allergies and dry eye

Some eye irritants exacerbate the effect of others on your eyes. For instance, if you have chronic dry eye, your eyes are much more likely to react to allergens. Having less of that protective tear film makes them more susceptible to surface irritants, Dr. Gans says.

The good news is that over-the-counter lubricating drops can make your eyes feel better, sometimes within minutes. Allergy drops can help reduce symptoms as well.

Dr. Gans “gently cautions” against using eye drops that claim to reduce redness, however.

They clear your eyes by shrinking surface blood vessels. But when they begin to wear off a rebound effect can occur. The blood vessels dilate even more and you need to use more drops to clear them again.

“You can get into a vicious cycle where you can’t get off the drops,” he says.

2. Broken blood vessels

Broken blood vessels on the eye’s surface (subconjunctival hemorrhage) may sound alarming, but they typically are painless and tend to clear up within a couple weeks without any treatment.

“There is no impact on your vision,” Dr. Gans says. “They are not worrisome, so just leave them alone and they will get better by themselves.”

Possible causes include:

• Trauma.
• A rapid increase in blood pressure.
• Blood thinners.
• Eye surgery.

3. Pink eye

Also known as conjunctivitis, pink eye occurs when viruses, bacteria or irritants such as chlorine cause inflammation and infection of the mucous membrane lining in the eyes.

Symptoms include itching, swelling, discharge and burning.

The condition can go away on its own. But, because it’s sometimes contagious, it’s probably best to see your doctor for medications you need to treat it. It can remain contagious for several days after exposure to the infection, Dr. Gans says.

4. Infection

Pain, light sensitivity and blurred vision are sometimes a sign of serious infection. You need to see a doctor within a couple of days, Dr. Gans says.

Bacteria, viruses, allergies, fungus or parasites (usually waterborne) all can cause infection.

If you wear contacts, and you suspect you have an infection, get to your doctor immediately. Untreated infections can cause erosion on the eye’s surface that can permanently damage your eyes.

“Contact lenses can cause infections and the chemicals used to sterilize them can cause irritation,” Dr. Gans says. “Contacts are a major source of potential infections that require immediate treatment.”

5. Uveitis

This condition also can produce unusual symptoms including light sensitivity, blurred vision and pain. It occurs when the iris and eye’s lining become inflamed. Without treatment, uveitis can cause vision loss, glaucoma or cataracts, so see your ophthalmologist right away.

If you experience any dryness, redness or unusual symptoms in your eyes for more than a day or so, see your eye doctor, Dr. Gans advises. An ophthalmologist can determine whether allergies, an infection or a more serious issue is at work.