Life's Little Mysteries

Why do coughs linger after a cold?

Young person of color preteen girl sneezes into her arm in pharmacy.
Why do coughs linger for so long? (Image credit: SDI Productions via Getty Images)

Colds and other respiratory illnesses are never fun. After the sneezing, sniffles and runny nose fade, one symptom often remains: coughing. But why do coughs sometimes take forever to go away? 

The main reason coughs are long-lasting is due to lingering inflammation, said Dr. Albert Rizzo, chief medical officer at the American Lung Association. This inflammation can have several sources, making it difficult to treat. 

These sources can include viral and bacterial infections that cause airway and nasal inflammation. This inflammation irritates the mucous membranes in the airway and nose and produces mucus — the phlegm and snot associated with colds. Nasal inflammation leads to postnasal drip, which is mucus that drips down the throat from the nose and is a common cause of coughing, according to the National Institutes of Health

When particles enter the airway through the nose or mouth they can trigger nerve receptors in the lungs to tell the brain, "That's not what we want down here," Rizzo said. Pressure then builds up in the diaphragm, and air is forcefully expelled, taking dust, food and mucus out with it. 

In addition to nasal inflammation, coughs linger after colds because airway inflammation may take a few weeks to subside, and this time could be extended if someone has had previous lung infections or is a smoker, Rizzo said. 

Related: Should you exercise if you're sick? 

When someone is sick, special immune cells called macrophages and neutrophils help fight infections in the airway. These cells are also inflammatory. Sometimes after a cold ends, these cells remain in the airway and keep it inflamed, which is why coughing can continue after infection, said Dr. Amy Dickey, a pulmonary and clinical care doctor at Massachusetts General Hospital (MGH) and an instructor of medicine at MGH and Harvard Medical School. 

In the meantime, delicate airway tissues can be extra sensitive to particles that enter through the nose or mouth. That’s because there's a complex system of nerves and muscles in the airway, throat and brain that control coughing. "Like if you [use a] reflex hammer on your knee, your leg kicks out. There can be similar types of reflexes in the airway," said Dickey. In other words, viruses and mucus are the reflex hammer and coughing is the kicking leg. Once inflammation goes down, this reflex becomes less sensitive and a cough should disappear.

A diagram of the human respiratory system. (Image credit: Inna Kharlamova via Getty Images)

For coughs that last three to four weeks after sickness there are a few home remedies and behaviors that can help shorten a cough's stay (or at least relieve symptoms). 

If postnasal drip accompanies a cough, nasal saline or nasal steroids can help reduce the inflammation contributing to the drip, Dickey said. These are often available over the counter. Throat lozenges can also help calm the larynx and suppress coughs, she added.

According to a 2021 study published in the International Journal of Cardiopulmonary Medicine and Rehabilitation, research shows that honey and saline can help relieve coughing. However, more studies are needed to confirm the efficacy and safety of natural products.  

While coughing can be nagging, it's crucial to remember that coughing serves an immune function. If irritants and mucus stay in the airways, they can damage the delicate airway tissues or the lungs, or even obstruct breathing. Dickey recommends exercising to stimulate deep breathing to loosen mucus, or taking expectorants that thin mucus and make it easier to cough up. This can help remove those inflammatory irritants.

While it's good to be considerate of other people, said Dickey, sometimes it’s important to let coughs out.   

While a lingering cough is most commonly caused by irritation from inflammation, people should see a doctor if a cough lasts more than three to four weeks and is accompanied by other symptoms like fever, shortness of breath or green and yellow mucus.

If a cough continues on its own for more than eight weeks, a doctor may want to do a chest X-ray or measure lung function to check for COPD, lung cancer, emphysema or other serious diseases, Rizzo said.

Hannah Loss
Live Science Contributor

Hannah Loss is a science journalist based in Boston. She covers the environment and has written for Scientific American, Sierra and Inside Climate News. Hannah graduated from Tufts University with a B.A. in English and environmental studies. She received a Master's degree in journalism from NYU's Science, Health and Environmental Reporting Program.