Overview

Smoking is the single-largest risk factor for lung-cancer worldwide, with as much as 80 percent of all lung-cancer cases coming from tobacco smokers. However, the remaining 20 percent of the cases come from people who do not currently smoke or smoked very little in the past (fewer than 100 cigarettes), as well as those who have never smoked in their life. So, what causes them to develop lung cancer? In this article, we will discuss why non-smokers are as much at risk of lung-cancer as smokers.

Introduction

As is well-known, a tumour or cancerous growth is uncontrolled growth of body-cells which rob healthy cells of their nutrition and hinder the normal functioning of cells, tissues and organs in the body. Changes in DNA of lung-cells can trigger the growth of cancerous tissue there. But it’s good to understand how the mechanism works.

DNA is important as they make up the chemical composition of our genes. We inherit genes from our parents and the DNA in them decides how we look. They also influence the risk or onset of various diseases including cancer in any part of the body.

Some of these genes help regulate or manage when new cells must be generated, when they must grow, divide or multiply, and finally die. Again, there are 2 types of such genes:

1. Oncogenes: These help cells to grow, multiply, and remain in the body.
2. Tumor-suppressor genes: These help aged or unhealthy cells to die at the right time, and also prevent unhealthy cell-division.

In some people, DNA changes (called mutations) in a certain part of the body turn-on the oncogenes while at the same time turning off the tumour suppressor genes. As a result, there is uncontrolled or abnormal growth of cells in that part of the body. When it comes to lung-cancer, there are several gene mutations that can increase the risk. We will examine them in a later section.

Also Read: Tobacco and Lung Cancer – The Deadly Connection

Types of non-smokers

Non-smoker is a generic word but there are actually 2 types of people here:

• Non-smokers: People who are currently not smoking. In the past they may have been heavy, moderate or light smokers (less than 100 cigarettes in total). They may have quit recently or a few years back.
• Never-smokers: Those who have never smoked tobacco in any form, even once, till date.

 Symptoms of lung-cancer in non-smokers

 The symptoms of lung-cancer are the same in both smokers and non-smokers:

• A cough that gets worse over time or doesn’t stop
• Chest pain or discomfort
• Trouble breathing
• Hoarseness
• Wheezing
• Coughing up blood
• Fatigue
• Recurring lung infections such as pneumonia
• Trouble while swallowing

If the cancer has spread (metastasized) to other parts of the body, the person may notice more symptoms such as:

• Poor appetite
• Weight loss for no reason
• Swelling in the neck, face or shoulder
• Blood clots
• Headache
• Fractures
• Pain in the bones

Types of lung cancer in non-smokers

There are different types of lung-cancers that affect non-smokers:

• Adenocarcinoma: This is a non-small-cell lung-cancer (NSCLC), and is the most common type, affecting almost 50–60% of lung-cancers in non-smokers. In this type, cancer starts in the lung cells that make mucus.
• Squamous cell carcinoma: This also is a type of NSCLC, and affects almost 10–20% of non-smokers. In this type, cancer starts in the cells that line the airways of the lungs. Incidentally, most smokers who develop lung-cancer have this type.
• Small-cell Lung cancer (SCLC): This is rarer and affects 6-8% of non-smokers.
• Other types, which make the balance of lung-cancer cases.

Also Read: Lung Biopsy- Procedure and Recovery

Risk Factors

Secondary smoke: Being constantly exposed to tobacco-smoke from a friend, partner, family-member or co-worker increases the risk of lung-cancer. In fact, a significant number of lung-cancer deaths in non-smokers, every year, comes from this category.

Radon: Radon is a radioactive metal that is naturally found in nature. When uranium, which is a heavier radioactive metal than radon starts decaying or decomposing, radon is one of the metals formed. Uranium present in the soil or water bodies is decaying constantly, so radon makes its way into the soil, into the water we consume, in the air we breathe, and in to homes that are constructed at such sites. That is why, in recent years, governments around the world are encouraging citizens to get radon testing for their homes and offices. In India too, several agencies that provide indoor air quality (IAQ) testing cover radon, among other parameters.

Toxic chemicals: There are various industries that use toxic chemicals like arsenic, chromium and asbestos for their products, so workers in such factories are at risk of developing lung-cancer over time. While asbestos was heavily used in the construction industry in the past, in recent years, its use is banned or severely restricted due to this risk. However, when it comes to other toxic chemicals, there is limited to no regulation. People who work at, or live close to – nuclear reactors are also at the risk from uranium that is used at these reactors. While there is some form of protection for workers, and restrictions for living close to a reactor, the risk is still there.

Air pollution: Excessive dust, smoke from industries, landfills where waste is dumped and burnt, and vehicle exhaust (especially diesel exhaust) increases the risk of lung-cancer for people who are constantly exposed to the same. People who live in highly polluted cities, neighbourhoods or industrial layouts should use good-quality face-masks. This is the best form of protection, as currently, lung-cancer screening is not done unless one has a history of smoking, or works in hazardous industries, or is showing any sign of lung-cancer.

The fifth major risk-factor is genetic changes which deserves a detailed mention.

Genetic changes

Certain gene mutations or changes happening in our DNA structure can increase the risk of lung cancer. These abnormal mutations can either be inherited from parents/grand-parents or acquired over time.

Inherited:

• A family history of lung cancer
• Inherited changes in chromosome 6 increases the risk, even if the person is a non-smoker or never-smoker.
• The body’s immune system has a certain ability to break down toxic chemicals found in tobacco smoke among other sources. Some people lose this ability which increases the risk of lung cancer for them. The same genetic changes can be inherited by their children/grand-children putting them at high risk.
• DNA can repair themselves when there is some damage to the body’s cells, and there are enzymes involved in this process. Some people have lower levels of this enzyme which increases the risk to them and their children/grand-children.
• The Epidermal Growth Factor Receptor (EGFR) is a protein that is a receptor for members of the EGF family of extracellular protein ligands. Mutations that affect their expression or activity is associated with non-small-cell lung cancer (NSCLC), especially in young, Asian women who do not smoke.

Acquired:

Genetic changes happening in the following genes can increase the risk of either SCLC or NSCLC.

• anaplastic lymphoma kinase (ALK)
• Kirsten rat sarcoma (KRAS) viral oncogene homolog
• RB1 tumour suppressor gene
• p16 tumour suppressor gene
• TP53 tumour suppression gene
• chromosome 3

Prevention

Consult a qualified doctor at a reputed hospital on how to reduce your risk of lung cancer in-spite of being a non-smoker or never-smoker. Broad guidelines include:

• If you live or work with a smoker, isolate yourself or that person, so that you are not exposed to second-hand smoke.
• If you live near a nuclear reactor, shift residence at the earliest.
• If you work in a hazardous industry or chemical industry that uses toxic chemicals, wear a good-quality face-mask and get an annual health check-up. Even if lung-cancer screening is not possible, there can be other signs that something is wrong.
• If you live in a polluted city or neighbourhood, wear a good-quality face mask and get annual health check-ups. The health check-up should include lung function tests such as spirometry.
• Engage a company/agency who does indoor air quality (IAQ) checks, for testing air quality at your home or office. Radon must be one of the parameters checked.
• Improve ventilation, and use air-purifiers, at homes and offices.

Reviewed by Dr Suresh S Venkita, Group Medical Director, Kauvery Hospitals

Kauvery Hospital is globally known for its multidisciplinary services at all its Centers of Excellence, and for its comprehensive, Avant-Grade technology, especially in diagnostics and remedial care in heart diseases, transplantation, vascular and neurosciences medicine. Located in the heart of Trichy (Tennur, Royal Road and Alexandria Road (Cantonment), Chennai, Hosur, Salem and Bengaluru, the hospital also renders adult and pediatric trauma care.

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