Everyone has already had red eyes, and probably more than once. It usually happens when the little tiny superficial blood vessels of the conjunctiva (the tissue that covers the white area of your eyes) increase and expand, causing it to turn reddish.

More often than not, red eyes are nothing that you should worry too much about, but you need to know how to identify when you’re dealing with a case worthy of medical attention.

Although rarer, there are a few serious eye conditions that could manifest through red eyes. Here’s everything you need to know

1. Common and benign causes

What exactly triggers that physical response? The causes can be many and from different origins.

First, we will discuss what the most common and less dangerous causes are, as well as their treatment.

Allergies: if besides having red, itchy, and crying eyes, you’re also frequently sneezing and with a stuffy nose, you are probably having an allergic reaction. Many people suffer a lot during springtime, when the levels of pollen released from grass and trees are at their highest, causing severe and really annoying allergies. Pets, dust, smoke, perfume, or mold could cause similar reactions. It’s important you know what you’re allergic to so that you can limit your exposure to allergy triggers (the most efficient form of treatment) or control the symptoms once you have them (antihistamines are commonly used to control pollen allergies).

Conjunctivitis: popularly known as “pink eye”, it refers to the inflammation of the interior of the eyelid and the exterior layer of the sclera (also known as the “white part of the eye”). Besides redness, your infected eye will also be itchy, have increased tears and seem to be stuck after waking up from a nap. This is not dangerous at all, and your body usually gets rid of it within 3 days.

However, in more complicated cases, there might be needed some antibiotics prescription – so you will need to see a doctor. You can help your body heal faster by using antihistamines or saline solution (in case the conjunctivitis origin is chemical). Prevention is the key, and all you need to do is make sure you wash your hands often and properly, as most cases are caused by bacteria that pass from your hand to your eye.

Tiredness: not having enough sleep can make your eyes red. That happens because it decreases the amount of oxygen that gets to your eye cells. To compensate for this change, your blood vessels dilate – hence the reddish color. The treatment is fantastic: take a nap.

Blepharitis: It is the most common cause of dry eye, and it is the inflammation of the oil glands that are located in the eyelids.

It’s a chronic condition, which means that there is no permanent treatment. However, it can, and it should be prevented and controlled – with good hygiene practices. Subconjunctival hemorrhage: although this condition may look really scary sometimes, there is absolutely no reason for alarm.

A subconjunctival hemorrhage occurs when a small vessel from the conjunctiva starts bleeding, leaking and spreading the blood across the white part of the eye. It is painless and usually caused by sneezing, coughing, vomiting, or rubbing the eye. Unless an eye infection starts developing, your eye will heal itself back to normal. You can ease the dryness and itchiness by applying artificial tears.

Dry eye: dry eye syndrome is another common disease that results as a condition of having dry eyes (when either the eye doesn’t produce enough tears or they evaporate much faster than normal). This could happen for several reasons (among which are the use of contact lenses, medication, pregnancy, and allergies).

Depending on the underlying cause, treatment may range from the use of artificial tears to changing certain medications.

Smoking weed: if you’re a regular smoker, you probably have already noticed that most people’s eyes turn red after smoking a joint. That’s because THC – the main psychoactive substance present in cannabis – decreases blood pressure, causing blood vessels to dilate. Having red eyes after smoking is nothing but a result of the dilation of the ocular capillaries. Don’t worry about it.

3. Dangerous causes

You should seek medical attention when, besides red eyes, you feel the following symptoms: reduced visual acuity, ciliary flush (ring of red spreading out from around the cornea), extreme pain, corneal or pupillary abnormalities, and abnormal intraocular pressure.

Let your doctor diagnose you and advise you about treatment options. These are usually symptoms of serious complications such as Keratitis: an inflammation or injury to the cornea that, if not properly treated, can become very dangerous. Keratitis could be chronic or infective (viral, bacterial, fungal, amoebic, or parasitic), with its treatment varying accordingly.

Glaucoma: this term refers to a comprehensive group of diseases that result in damage to the optic nerve and directly affect vision. Some of them (acute glaucoma, for example) could even eventually lead to permanent vision loss. Scleritis: this is an inflammatory disease that attacks the sclera.

It usually causes pain, and could also lead to permanent vision loss. Iritis: iritis is caused by the inflammation of the uvea (the uvea involves the ciliary body, the iris, and choroid). This condition is very dangerous, as shown by the fact that, in 2004, it was responsible for more than 10% of the blindness in the United States, according to a study published by the magazine “Ophthalmology”.

Eye injuries: in case you suffered some accident that affected your eyes, you should go immediately to the hospital. If not properly treated (or if treatment is delayed), physical or chemical injuries of the eye could seriously threaten your vision.

Also, if your workplace requires the use of protection glasses, always use them!