Understanding lung cancer and its causes

In Australia, lung cancer is now the fifth most commonly diagnosed cancer–with an estimated 15,100 people expected to receive a diagnosis of lung cancer in 2024.¹ Sadly, lung cancer is also the leading cause of cancer-related deaths in both women and men.¹ Here, we’ll take a closer look at lung cancer, its causes, and potential ways to help reduce the risk of developing the disease.

Lung cancer occurs when abnormal cells grow in part of the lung tissue and then continue to multiply in an uncontrolled way.² Lung cancers can be described in different ways and may be called by a different name–these terms just help to give more information about the size and type of cancerous cells and where they are located.

Primary cancer describes the site where a cancer first starts–so when a cancer first starts in the lungs, it is called primary lung cancer. When a cancer spreads from its original site to another site, it is called a secondary cancer or a metastasis.² So metastatic lung cancer describes cancer that has spread (or metastasised) from the lungs to somewhere else in the body, while secondary lung cancer describes cancer that has spread (or metastasised) from its primary site somewhere else in the body to the lungs.³

Lung cancer is typically classified as one of two main types, which is based on the size of the cancer cells. Around 90% of lung cancers are called non-small cell lung cancer (NSCLC), while the other 10% are called small cell lung cancer (SCLC).⁴

Within NSCLC, there are three main subtypes, which are named based on the type of cells within the lungs where the cancer starts to develop:^2,4

• Adenocarcinomas develop in the mucus-secreting cells that are present in the deep part of the lungs
• Squamous cell carcinomas develop in the cells that line the large airways in the lungs
• Large cell undifferentiated carcinomas may develop in more than one type of cell

While the causes of lung cancer are not completely understood, there are a number of risk factors known to be associated with an increased risk for developing lung cancer. These factors can be divided into modifiable factors – things that can be changed to decrease risk – and non-modifiable factors – things that are set and can’t be changed.^2,4 We’ll review the important modifiable risk factors first, and then look at some of those non-modifiable risk factors that are out of our control.

It’s well established that current or former tobacco smoking is the greatest risk factor for developing lung cancer, with cigarette smoking thought to account for an estimated 70% to 90% of all lung cancers.⁵ This risk increases with the length of time a person has smoked for and the number of cigarettes smoked per day.^2,5

The good news here is that stopping smoking – or even reducing the amount of smoking – has been shown to reduce the risk of developing lung cancer, with a greater risk reduction associated with a longer time since quitting.⁵ That said, not everyone who does or has smoked will get lung cancer and some people who have never smoked will also get lung cancer,² which demonstrates that the causes of lung cancer are complex.

What about other forms of ‘smoking’–does smoking e-cigarettes (vaping) or marijuana cause lung cancer? Unfortunately, there is a lack of good quality data to show how these activities may be associated with lung cancer.⁵ Even when these products don’t contain tobacco, smoking e-cigarettes or marijuana may cause damage to the lungs – among other harmful health effects.^5,6

Other modifiable risk factors identified as increasing the risk for developing lung cancer include exposure to a range of environmental or occupational (work-related) factors, some of which are more impactful in people who smoke tobacco:^2,5

• Second-hand smoke (passive smoking) – breathing in tobacco smoke from others can increase the risk of lung cancer in people who don’t smoke by up to 30%;^§ again, more exposure means greater risk, and exposure for some people may begin at a very young age^2,5
• Asbestos – exposure to asbestos, especially certain types of asbestos fibres, can increase the risk of lung cancer, as well as mesothelioma (a type of cancer that affects the outer covering of the lung, called the pleura)^2,5
• Other occupational substances – exposure to a range of chemicals (e.g., arsenic, formaldehyde), diesel exhaust, silica, and radon (a radioactive gas) are associated with increased risk of lung cancer^2,5
• Air pollution – exposure to outdoor air pollution as well as indoor smoke from cooking and heating can also be a risk factor for lung cancer^2,5

If you have a family member who has or has had lung cancer, you may be wondering if lung cancer is hereditary. While the precise role of genetic factors is not well understood, it has been shown that having a first-degree relative (parent, sibling, or child) with lung cancer can increase the risk for developing lung cancer.⁵

While lung cancer can affect people of any age, it is most often diagnosed in people aged over 60 years.^1,2

The risk of lung cancer may be increased in people who have a lung disease (e.g., pulmonary fibrosis, chronic obstructive pulmonary disease) or human immunodeficiency virus (HIV).²

It’s important to remember that having one or more of these risk factors doesn’t mean that a person will go on to develop lung cancer. If you have any questions or concerns about lung cancer and its risk factors, please talk to your doctor or specialist.