Croup is a childhood respiratory infection that causes swelling in the upper airway, producing a distinctive bark-like cough that most parents recognize immediately. It primarily affects children between 6 months and 3 years old, hits boys slightly more often than girls (at roughly a 1.4 to 1 ratio), and is responsible for a significant share of emergency room visits during fall and winter months. Most cases resolve on their own within a few days, but understanding what’s happening and what to watch for can save you a panicked night.
What Happens Inside the Airway
Croup targets the voice box and the area just below it, where a child’s airway is already narrow. When a virus infects this area, the tissue swells inward, shrinking the opening the child breathes through. That narrowed passage is what creates the two hallmark sounds of croup: a harsh, seal-like barking cough and a high-pitched whistling noise when the child breathes in (called stridor). The child’s voice often sounds hoarse, and they may have a low fever and runny nose in the days before the cough appears.
Because young children have small, soft airways, even a small amount of swelling can significantly reduce airflow. This is why croup is overwhelmingly a disease of toddlers and preschoolers. By the time children are five or six, their airways have grown large enough that the same virus might cause nothing more than a sore throat or cold.
What Causes It
Parainfluenza virus, mainly types 1 and 2, is responsible for more than two-thirds of all croup cases. It spreads the same way most respiratory viruses do: through droplets from coughing and sneezing, or by touching contaminated surfaces and then touching the face. Other viruses that can trigger croup include influenza A and B, respiratory syncytial virus (RSV), adenovirus, rhinovirus, and SARS-CoV-2. Bacterial infections cause croup far less often but tend to be more severe when they do.
Croup season peaks in the fall, roughly tracking with parainfluenza circulation, though cases can appear year-round depending on which virus is driving them.
Why Symptoms Get Worse at Night
One of the most unsettling things about croup is how quickly it can escalate after bedtime. A child who seemed mostly fine during the day may wake up in the middle of the night struggling to breathe. This happens for a few reasons. Cortisol, the body’s natural anti-inflammatory hormone, drops to its lowest levels overnight, allowing airway swelling to worsen. Lying flat also lets mucus pool in the already narrowed airway. And the cooler, drier air of nighttime can irritate inflamed tissue further.
Symptoms typically peak on the second or third night of illness and then gradually improve. The full course usually lasts three to seven days, though the barking cough can linger a bit longer.
Mild, Moderate, and Severe Croup
Not all croup episodes are equal, and knowing where your child falls on the severity spectrum helps you decide what to do next. Doctors use a scoring system based on five factors: the presence of stridor, visible pulling-in of the skin around the ribs or breastbone (called retractions), skin color changes, alertness, and how well air is moving in and out of the lungs.
Mild croup means the child has the barking cough and possibly some stridor when upset or crying, but breathes comfortably at rest. This accounts for the majority of cases and can usually be managed at home.
Moderate croup involves stridor even when the child is calm, visible retractions, and increased restlessness. These children generally need medical evaluation and often benefit from treatment to reduce the swelling.
Severe croup is marked by significant breathing difficulty, pronounced retractions, blue-tinged skin (especially around the lips and fingertips), and sometimes lethargy or disorientation. Severe croup can be life-threatening and requires immediate emergency care.
How Croup Is Treated
For mild cases, the most effective thing you can do at home is keep your child calm. Crying and agitation increase the effort of breathing through a narrowed airway, which makes symptoms worse and can create a cycle of panic for both child and parent. Sitting with your child, reading a story, or holding them upright can help more than you might expect.
You may have heard that steam from a hot shower or cool night air can ease croup symptoms. A systematic review of the evidence found that humidified air inhalation did not significantly improve croup scores in children with mild to moderate symptoms. It’s not harmful, and some parents feel it helps in the moment, but the data doesn’t support it as a reliable treatment. Cool air exposure on a winter night may offer brief relief, likely by reducing swelling the way a cold compress reduces a swollen ankle, but the effect is temporary.
When croup is moderate or severe, the standard treatment is a single dose of a corticosteroid, which reduces airway swelling. The effect takes a few hours to fully kick in, but it can dramatically shorten the course and prevent the illness from worsening. For children in significant respiratory distress, doctors also use a nebulized form of epinephrine, which works within about 30 minutes by constricting the swollen blood vessels in the airway. The relief from nebulized epinephrine is real but short-lived, typically wearing off within two hours. That’s why children who receive it are usually monitored in the emergency department for a period afterward to make sure symptoms don’t rebound.
Signs That Need Emergency Attention
Most croup is scary-sounding but manageable. The signs that should send you to an emergency room are stridor that doesn’t stop when the child is resting quietly, visible retractions where the skin sucks in sharply between or below the ribs with each breath, any blue or gray discoloration around the lips or fingernails, drooling or difficulty swallowing, and unusual sleepiness or confusion. A child who is sitting very still and leaning forward to breathe, or who seems to be working hard with each breath, needs immediate evaluation.
If your child has had repeated episodes of croup, or if croup-like symptoms appear in a child younger than 6 months or older than 6 years, it’s worth discussing with a pediatrician. Recurrent or atypical episodes sometimes point to an underlying airway issue rather than simple viral croup.