Coughing starts when sensory receptors lining your airways detect something that shouldn’t be there, whether that’s mucus, dust, acid, or inflammation from an infection. These receptors send signals through the vagus nerve to a “cough center” in your brainstem, which coordinates the explosive burst of air that clears the irritant. This reflex arc is one of the body’s most important defense mechanisms, but it can also misfire, becoming persistent and disruptive when the underlying trigger isn’t resolved.
How the Cough Reflex Works
Cough receptors are concentrated in the trachea, the point where the main airways branch, and the lining of larger airways deeper in the lungs. But they also exist in less obvious places: the throat, the ear canals, the diaphragm, the lining around the heart, and even the stomach. That’s why an ear infection can make you cough, or why acid in your esophagus can trigger a fit that feels like it’s coming from your lungs.
These receptors come in two main types. Chemical receptors respond to acid, heat, and compounds similar to capsaicin (the molecule that makes chili peppers burn). Mechanical receptors respond to physical touch or stretching, like when mucus pools in an airway or a piece of food brushes the back of your throat. Once triggered, the signal travels along the vagus nerve to the brainstem, which orchestrates the deep inhale, the brief closure of your vocal cords, and the sudden pressurized blast of air that is a cough. Higher brain centers can also influence this process, which is why you can suppress a cough in a quiet room or why stress and anxiety sometimes make coughing worse.
Viral Infections: The Most Common Trigger
The common cold and flu are far and away the most frequent reasons people cough. When a virus infects the cells lining your airways, it sets off a cascade of inflammation. Your immune system releases inflammatory molecules, and the infected tissue ramps up mucus production by activating mucus-producing genes. That extra mucus directly irritates cough receptors.
But mucus is only part of the story. Viral infections also cause the release of substances called tachykinins, particularly substance P, which are potent nerve-stimulating molecules. These cause airway muscles to contract, blood vessels to dilate, and even more mucus to be secreted. At the same time, the enzymes that normally break down these stimulating molecules become less active during infection, so the effect is amplified. The virus essentially turns up the volume on every signal that triggers coughing while also turning down the body’s ability to quiet those signals. This is why a cough from a cold can linger for weeks after the infection itself has cleared: the inflammatory changes take time to fully resolve.
Post-Nasal Drip and Upper Airway Irritation
When mucus drains from the sinuses or nasal passages down the back of the throat, it can irritate the cough receptors concentrated there. This is formally called upper airway cough syndrome, and it’s one of the top three causes of coughs that last eight weeks or longer. Patients typically describe the feeling of something stuck in the throat, along with frequent throat clearing and a dry, persistent cough.
The mechanism isn’t entirely about the physical drip of mucus. Inflammation in the upper airway itself may directly sensitize cough receptors, making them fire more easily. Allergies, chronic sinus infections, and irritation from dry air can all keep this cycle going. A doctor looking for this pattern might notice a “cobblestone” texture on the back of the throat, which is a sign of chronic mucus drainage and irritation.
Acid Reflux Without Heartburn
Acid reflux is a surprisingly common cause of persistent cough, and roughly 70% of people whose cough is driven by reflux don’t experience typical heartburn or a sour taste. This “silent” reflux triggers coughing through two pathways. First, refluxed acid or gas can travel high enough to directly contact the throat and lower airway receptors. Second, and more subtly, acid in the esophagus stimulates nerve endings there that share wiring with the airways through the vagus nerve, triggering a cough reflex even though nothing has actually reached the lungs.
The reflux doesn’t even need to be strongly acidic. Weakly acidic reflux can activate mechanical stretch receptors in the esophagus, and repeated exposure of any kind can lead to a state of heightened nerve sensitivity. Over time, inflammatory molecules like histamine and substance P accumulate around nerve endings, lowering the threshold for a cough to fire. This is why reflux-related coughs are notoriously persistent: the nerves themselves have become more reactive.
Cough-Variant Asthma
Asthma doesn’t always involve wheezing or shortness of breath. In cough-variant asthma, a dry, nonproductive cough lasting more than eight weeks is the only symptom. Lung function tests often come back normal, but the airways are hyperreactive, meaning they tighten in response to triggers that wouldn’t bother most people, like cold air, exercise, or allergens. The cough is frequently worse at night.
What distinguishes cough-variant asthma from other causes is that it responds to the same treatments used for typical asthma: inhaled bronchodilators and corticosteroids. The underlying problem is eosinophilic inflammation, where a specific type of immune cell accumulates in the airway walls, keeping them in a state of chronic irritability. Without treatment, cough-variant asthma can progress to typical asthma with wheeze and breathlessness in some patients.
Blood Pressure Medications
A class of blood pressure drugs called ACE inhibitors causes a dry, tickling cough in 5% to 39% of people who take them. The cough can start within days of beginning the medication or develop months later, which makes it easy to miss the connection. ACE inhibitors work by blocking an enzyme involved in blood pressure regulation, but that same enzyme is responsible for breaking down certain substances in the lungs, including bradykinin and substance P. When those substances accumulate, they sensitize the cough receptors. The cough typically resolves within one to four weeks after switching to a different type of blood pressure medication.
Environmental Irritants and Secondhand Smoke
Particulate matter, cigarette smoke, strong perfumes, cleaning products, and occupational dust all activate a specific receptor channel in airway nerves called TRPA1. This channel responds to a wide range of irritant chemicals found in air pollution and smoke, including acrolein and other combustion byproducts. It essentially functions as an oxidative stress sensor, detecting cellular damage caused by inhaled toxins.
The effects are cumulative. In a study of nearly 12,000 children in an industrial region of China, exposure to suspended particulates, sulfur dioxide, and nitrogen dioxide increased the risk of persistent cough by 21% to 28%. Secondhand smoke has an especially long reach: a study following roughly 35,000 nonsmokers found that those who lived with a smoker before age 18 had more than double the risk of chronic dry cough later in life, even after years of no further exposure. Cold air, talking, and eating can also trigger coughing in people whose airways have become hypersensitive from chronic irritant exposure.
When Nerves Themselves Are the Problem
In some people with chronic cough that doesn’t respond to standard treatments, the problem isn’t an ongoing irritant. It’s that the nerves controlling the cough reflex have become permanently more sensitive, a condition now understood as cough hypersensitivity syndrome. Prior infections, allergies, or prolonged inflammation can damage or rewire sensory nerves in the larynx and airways, lowering the threshold for triggering a cough to the point where normal sensations like a deep breath, a change in temperature, or even speaking are enough to set it off.
This is essentially the same process that causes chronic pain after a nerve injury. The nerves become hyperexcitable and begin firing at stimuli that wouldn’t normally register. Supporting this idea, medications originally developed for nerve pain, like gabapentin, have been shown in randomized trials to reduce cough frequency in patients with this type of refractory cough. The recognition that chronic cough can be a neuropathic condition has been a significant shift in how it’s understood and treated.
Acute, Subacute, and Chronic Cough
Duration is one of the most useful clues to what’s causing a cough. An acute cough lasts fewer than three weeks and is almost always caused by a viral upper respiratory infection, though it can also signal something more urgent like pneumonia or a blood clot in the lung. A subacute cough, lasting three to eight weeks, is most commonly a post-infectious cough where the virus is gone but the airway inflammation hasn’t fully settled. A chronic cough, persisting beyond eight weeks, points toward conditions like asthma, reflux, upper airway cough syndrome, or medication side effects.
Signs That Need Prompt Attention
Most coughs resolve on their own, but certain accompanying symptoms change the picture. Coughing up blood or pink-tinged mucus, significant shortness of breath, chest pain, fainting, or difficulty swallowing warrant immediate evaluation. A cough paired with a high fever, thick greenish-yellow phlegm, unexplained weight loss, or ankle swelling also signals something beyond a routine infection. Any cough that persists beyond a few weeks without improvement is worth investigating, particularly because the most common chronic causes, including reflux and cough-variant asthma, are highly treatable once correctly identified.