Lung nodules are small, round spots in the lungs, each measuring less than 3 centimeters (about 1.2 inches) across. They show up on roughly 30% of all chest CT scans, which means they’re extremely common and usually found by accident during imaging for something else entirely. The vast majority are benign, but understanding what causes them helps explain why your doctor may recommend monitoring or further testing.
A nodule can form from a past infection, ongoing inflammation, environmental exposure, or, less commonly, cancerous growth. The cause often determines what the nodule looks like on a scan, how it behaves over time, and what happens next.
Past and Current Infections
Infections are one of the most frequent causes of lung nodules. When your immune system fights off a lung infection, it sometimes walls off the area of damage into a small, dense cluster of immune cells called a granuloma. That granuloma can linger for years or even decades after the original infection has cleared, showing up on a CT scan long after you’ve forgotten you were ever sick.
Tuberculosis is the most common infectious cause of granulomas worldwide. In the United States, fungal infections are a particularly common culprit, especially in certain regions. Histoplasmosis, caused by a fungus found in soil contaminated with bird or bat droppings, is widespread in the Ohio and Mississippi River valleys. Valley fever (coccidioidomycosis) is common in the desert Southwest. Blastomycosis tends to appear in the Great Lakes and Ohio River regions. Many people with these infections experience mild or no symptoms and never realize they were exposed until a nodule appears on imaging years later.
Other infections that can leave behind nodules include aspergillosis, cat scratch disease, and certain viral infections like cytomegalovirus and Epstein-Barr virus. Parasitic infections, though rarer in North America, can also cause them.
Inflammatory and Autoimmune Conditions
Your immune system can also create nodules when it misfires, attacking your own tissue or reacting to an unknown trigger. Sarcoidosis is a classic example. In this condition, the immune system forms granulomas in the lungs (and sometimes other organs) in response to what researchers believe is an unidentified antigen in genetically susceptible people. The typical pattern involves clusters of immune cells accumulating in lung tissue, often affecting the lymph nodes between the lungs as well.
Rheumatoid arthritis can produce lung nodules, particularly in people with more severe joint disease or high levels of a specific antibody called rheumatoid factor. These nodules are made of the same inflammatory tissue found in the joint lining. Other inflammatory conditions linked to lung nodules include granulomatosis with polyangiitis and certain types of vasculitis.
Environmental and Occupational Exposures
Breathing in certain dusts over months or years can cause the lungs to form nodules as a protective response to particles that can’t be broken down or expelled. Crystalline silica is one of the best-known offenders. Workers in mining, sandblasting, stone cutting, and construction face the highest exposure. Silica particles trigger inflammation and scarring in the lungs, a condition called silicosis, which often shows up as multiple small nodules on imaging. CT scans are more sensitive than standard chest X-rays at detecting these changes.
Asbestos exposure can also produce lung nodules, along with thickening of the tissue lining the lungs. Coal dust, beryllium, and certain metal dusts carry similar risks. These nodules typically develop after years of repeated exposure rather than a single event.
Benign Tumors
Not every nodule comes from infection or inflammation. Some are benign (non-cancerous) growths. The most common is a hamartoma, which accounts for an estimated 77% of benign lung nodules. Hamartomas are slow-growing lumps made of normal tissue types like cartilage, fat, and connective tissue that have simply grown in a disorganized way. They’re typically smaller than 4 centimeters, appear as well-defined, coin-shaped spots on imaging, and almost never cause symptoms. Most are found in the outer portions of the lung and require no treatment.
Cancer-Related Nodules
A small percentage of lung nodules turn out to be cancerous. The risk depends heavily on size. Nodules smaller than 6 millimeters carry a very low probability of malignancy, while those larger than 8 millimeters warrant closer evaluation. Other factors that raise concern include older age, a history of smoking, a family history of lung cancer, and whether the nodule has irregular or spiky edges on the scan.
A cancerous nodule can be a primary lung cancer, meaning it started in the lung, or a metastasis, meaning cancer from another part of the body (such as the colon, breast, or kidney) has spread to the lungs. Metastatic nodules often appear as multiple round spots rather than a single one.
How Doctors Tell the Difference
The appearance of a nodule on a CT scan offers important clues. Features that suggest a nodule is benign include smooth, well-defined edges, dense calcification (especially in a central or layered pattern), and stability over time. Features that raise suspicion for malignancy include spiky or irregular margins, rapid growth between scans, and a hazy, ground-glass appearance, which paradoxically can sometimes indicate a higher malignancy potential even when the nodule looks faint.
Size plays a major role in the next steps. For solid nodules smaller than about 6 millimeters in people without major risk factors, guidelines generally recommend no routine follow-up. Larger nodules, particularly those over 8 millimeters, may call for a repeat CT scan in about three months or a PET scan. PET scans, which detect metabolic activity, are most accurate for nodules larger than 10 millimeters, where their sensitivity for identifying cancer reaches about 95%. For nodules between 5 and 10 millimeters, sensitivity drops to around 69%, which is why small nodules are typically monitored with repeat imaging rather than immediately biopsied.
When a combined PET-CT scan is used, the overall sensitivity for detecting malignancy reaches roughly 97%, with a specificity around 85%. That means it’s very good at catching cancers but occasionally flags benign nodules as suspicious, which is why biopsy remains the definitive answer when imaging is inconclusive.
What Follow-Up Looks Like
If your doctor finds a lung nodule, the follow-up plan depends on its size, appearance, and your personal risk profile. For many small, solid nodules, the plan is simply a repeat CT scan in 3 to 12 months to check whether the nodule has changed. A nodule that stays the same size over two years is almost certainly benign. Subsolid nodules, those that appear partly hazy on the scan, are monitored on a slightly different schedule because some slow-growing cancers can look this way.
For larger or suspicious nodules, your doctor may recommend a PET-CT scan, a needle biopsy guided by CT imaging, or in some cases a bronchoscopy. The goal is always to determine whether the nodule is something that needs treatment or something your body created long ago and left behind.