If you suddenly get congested and this rarely happens to you, the most likely cause is a virus — probably the common cold. Colds are very contagious and can happen any time of year, though they are more common in winter. Not all colds feel the same. They can be mild or severe, and they can start in the nose, the throat, or the chest and then spread to other areas.

Clues that you have a virus (a cold):

It came on suddenly and you feel a little tired
Lots of nasal drainage and a lot of blowing your nose
Usually no pain
Heavy congestion and a blocked feeling
Usually both sides of your nose are affected
People you've been around are also sick
Symptoms usually start to improve by day 5 or 6 and are gone within two weeks
Allergy medicines (antihistamines) don't help much

Other causes of sudden congestion include allergy flare-ups and bacterial sinus infections.

Allergy flare-ups are often seasonal, set off by a specific pollen bloom — or by an exposure such as digging in mulch and stirring up mold spores, going through dusty boxes, or being near a pet you are allergic to. Allergy reactions usually cause sneezing, itchy eyes, and lots of clear runny drainage. They typically settle down within a couple of days once you get away from whatever set them off.

If your sudden congestion is rare but happens at the same time every year, think about seasonal allergies. These flare up when you are allergic to a specific pollen and that pollen suddenly blooms. Ragweed is one of the most common, but many others can do it. Sudden pollen-related allergy flare-ups are sometimes called "hay fever." Hay fever isn't usually caused by hay, and there is no fever — it's just an old name for a sudden run of sneezing, congestion, itchy eyes, and sometimes a cough or sore throat tied to a pollen bloom.

Clues that your problem is allergies:

It builds over a few days unless you had a clear exposure (like cleaning a musty garage or doing yard work)
Sneezing, itching, and congestion are the main symptoms
Usually no pain
Itchy eyes are common
Usually both sides of your nose are affected
You haven't been around anyone who is sick
Symptoms come and go depending on whether you are indoors or outdoors
Allergy medicines (antihistamines) clearly help

Bacterial sinus infections often come on after a cold or an allergy flare-up. They are often only on one side and often cause facial pain or sinus headaches. The drainage is often cloudy or colored, and in children a cough is the most common symptom.

Clues that you have a bacterial sinus infection:

A cold or allergy attack seemed to be getting better, then suddenly got worse again
One side of your face is more affected than the other
Pain in your sinuses, your face, or your upper teeth
Thin yellow or green drainage
Not getting better after a week
You sometimes notice a bad smell
Headaches in someone who doesn't usually get them
A cough from drainage running down the back of the throat

What should you do?

If this sounds like you, first think about whether it could be a virus and look at the section on treating colds with over-the-counter medicines. People often underestimate how many colds they actually get and quickly assume it's allergies or something else.

If you are pretty sure it isn't a cold, try over-the-counter allergy medicines for about a week. If they help, some can safely be used long-term. If they don't help, it's time to see a doctor. If this keeps happening and over-the-counter medicines aren't working, you need a proper diagnosis.

Bacterial infections and stubborn allergies often need prescription medicines. 

When you get congested often but it doesn't stay for long, it is usually from one of three things: allergies, flare-ups of long-term sinusitis, or frequent colds. Let's look at how to tell these apart.

Frequent colds (viral infections)

Some people just catch a lot of colds. You feel fine, then suddenly you get congested, your nose is blocked, you have clear runny drainage, and sometimes a sore throat or a cough. Adults usually don't get a fever. There are hundreds of cold viruses, and each one can feel a little different. Cold symptoms almost always affect both sides of your nose. You may also feel generally run-down. Real face pain is uncommon. The drainage is usually clear, but it can sometimes look slightly cloudy.

The average adult gets about two colds a year. People who are around young children get more — schoolteachers, daycare workers, and parents of young kids are at the highest risk. Sometimes you can have a run of bad luck and catch one cold right after another. That can look a lot like a single long-term problem, such as a bacterial sinus infection. If you catch colds often, it can really help to understand how viruses spread and take some simple steps to avoid them.

Allergies (allergic rhinitis)

Allergies are a common cause of congestion, and for some people this happens often. Along with congestion, you may have sneezing, itchy eyes, and clear runny drainage. Allergies happen when something you breathe in (a particle you are allergic to) causes inflammation on the lining of your nose. What you are allergic to determines when you have symptoms and how bad they are. About 20–40% of people have some form of allergic rhinitis. Not everyone has severe or frequent symptoms. Reactions to cigarette smoke, fumes, and strong odors are usually not true allergies — they are direct irritation — but they can feel similar. These triggers are usually obvious.

If you are allergic to mold spores or dust mites, you can have reactions any time of year. Big flare-ups happen when you get exposed to a lot of the trigger at once. Dust mites are especially common in carpet, bedding, and curtains. Mold spores build up in houseplants, flower beds, old magazines and books, items kept in damp places, and anywhere that is often damp and poorly ventilated.

If your congestion comes back at the same time(s) every year, think about seasonal allergies. These flare up when a specific pollen suddenly blooms. Ragweed is one of the most common, but many others can do it. This kind of sudden pollen reaction is sometimes called "hay fever" — even though it isn't usually caused by hay and there's no fever.

Long-term sinusitis (chronic sinusitis)

Chronic sinusitis is one of the most common causes of constant congestion, but it can also cause congestion that comes and goes in waves. We don't fully understand why these flare-ups happen. The medical name is "acute flare-ups of chronic sinusitis." When this flares up, one side is often worse than the other, and sinus pain and sinus headaches are common. Chronic sinusitis can be caused by low-level bacterial infection, inflammation triggered by inhaled mold spores, or physical blockage of the sinus drainage paths — for example by polyps, cysts, or other unusual anatomy.

What should you do?

It can be hard to tell these three causes apart. If you have young children or work around children, at least some of these episodes are probably colds — try treating them that way first. If you are pretty sure it isn't colds, try over-the-counter allergy medicines for about a week. If they help, some are safe for long-term use. Steroid nasal sprays used every day are the single most effective medicine for nasal allergy symptoms. Look at the section on treating long-term problems with over-the-counter medicines. If things still aren't going well, see a doctor. If this keeps happening and over-the-counter medicines aren't enough, you need a proper diagnosis.

Tips for avoiding colds

Colds spread when tiny amounts of nasal mucus or saliva get passed from one person to another. Droplets from coughs and sneezes are one way. But the most common way is touching something a sick person has touched, then touching your own mouth, nose, or eyes.

If you want to catch fewer colds — especially when you are around sick people, in crowds, or during cold season — try not to touch your face with your hands. If you need to scratch or rub your face, use a tissue or cloth. Wash your hands often or use hand sanitizer. People rub their eyes more often than they realize; when you need to, use a tissue or the inside of your shirt instead of your bare hand. Simple steps like these can really cut down on how often some people get sick — especially teachers, parents of young children, and anyone whose job involves a lot of contact with the public.

Nasal congestion lasting more than 6 weeks

Long-term sinusitis (chronic sinusitis) is probably the most common cause of frequent or constant congestion. It is exactly what it sounds like — long-term inflammation of the sinus cavities. It can be caused by a low-level bacterial infection that won't go away, ongoing inflammation from breathing in mold spores, or unusual anatomy that gets in the way of normal drainage. There are other, less common causes too.

Allergies

Allergies can cause constant congestion. Year-round allergies are called perennial allergic rhinitis. For symptoms to last all year, the trigger has to be present all year. Common year-round triggers include dust mites, mold spores, pet dander, and cockroach particles.

Unusual anatomy inside the nose

Sometimes constant congestion is purely a structural problem. Enlarged adenoids (lymph tissue at the back of the nose), enlarged lower turbinates, a concha bullosa (an air bubble in a nasal bone), and a crooked septum can all cause it. To find out if any of these are involved, you usually need to be examined by a specialist.

What should you do?

If this sounds like you, start with the sections on treating allergies and treating long-term problems with over-the-counter medicines. If you haven't tried those steps yet, give them a try.

If that doesn't help, see your doctor. If you have had this for 6 weeks and over-the-counter medicines aren't working, it's time to find out what's going on. If nothing seems to help, you may have chronic sinusitis and should see a specialist.

How chronic sinusitis is diagnosed

Chronic sinusitis is first diagnosed based on your symptoms. Later, a CT scan is usually done to confirm it. The starting checklist of symptoms doctors look for includes —

Two or more of these main symptoms:

Pain or pressure in the face
A blocked or stuffy nose
Nasal discharge, pus, or discolored drainage going down the back of the throat
A reduced or absent sense of smell
Pus seen inside the nasal cavity

Or one of the above plus two of these:

Headache
Fever
Bad breath
Fatigue
Tooth pain
Cough
Ear pain, pressure, or fullness

Note: face pain by itself isn't enough — it has to come with at least one of the other main symptoms above.

The official 2003 criteria for long-term sinusitis (chronic rhinosinusitis) require these symptoms for more than 12 weeks. In addition, the doctor needs to see at least one sign of inflammation, such as:

Discolored drainage, nasal polyps, or polyp-like swelling found during a nasal exam with a scope (after a decongestant spray has been used)
Swelling or redness inside the nose, seen on a scope exam
Areas of redness, swelling, or inflamed tissue in the nose (if these aren't seen, an imaging study is needed to confirm the diagnosis)

Imaging that can confirm the diagnosis includes —

A CT scan showing thickened lining, changes in the bone, or fluid levels in the sinuses

OR

A plain sinus X-ray showing fluid levels or more than 5 mm of cloudiness in one or more sinuses

MRI is not usually used for this diagnosis because it picks up too many findings that don't matter clinically.

In general, plain X-rays don't reliably show sinus disease. CT scans are considered the best imaging test for evaluating chronic sinusitis.

When the congestion is mostly on one side

Some causes are less likely if only one side is affected. Allergies and colds usually affect both sides of the nose about equally.

If your one-sided congestion is long-term, the possibilities are very similar to those for long-term congestion on both sides.

Unusual anatomy is a common cause of long-term one-sided congestion. Examples include a crooked septum, cysts, polyps, and tumors. Often, the anatomy works together with an infection or allergies to make one side worse.

A bacterial sinus infection can cause one-sided congestion, especially if it came on recently and you also have face pain or upper-tooth pain.

Colds can start on one side, but they usually spread to both sides within a couple of days. If you have a crooked septum or other unusual anatomy, a cold may feel worse on one side.

A few things to think about

If the air feels blocked on one side, a crooked septum is worth considering — especially if there isn't much drainage and the problem has been steady for a long time.

If symptoms linger on one side after an illness, think about long-term sinusitis on that side.

Mild long-term symptoms with pressure between the eyes can come from a concha bullosa, a crooked septum, a low-level long-term infection, or sometimes from certain types of headaches.

What should you do? If this sounds like you, start with the section on treating long-term problems with over-the-counter medicines. Try those steps if you haven't already.

If that doesn't help, see your doctor. If you have had this for 6 weeks and over-the-counter medicines aren't working, it's time to find out what's going on. Chronic sinusitis and a crooked septum usually need a specialist to diagnose and treat properly.