- Describe how carcinogens cause cancer and how cancer can be treated or prevented.
- Identify causes of air pollution and its effects on human health.
- Explain how bioterrorism threatens human health.
Air Quality Index (AQI)
assessment of the levels of pollutants in the outdoor air that is based on their human health effects
intentional release or spread of agents of disease
anything that can cause cancer
Besides pathogens, many other dangers in the environment may negatively affect human health. For example, air pollution can cause lung cancer. It can also make asthma and other diseases worse. Bioterrorism is another potential threat in the environment. It may poison large numbers of people or cause epidemics of deadly diseases.
Carcinogens and Cancer
A carcinogen is anything that can cause cancer. Cancer is a disease in which cells divide out of control. Most carcinogens cause cancer by producing mutations in DNA.
Types of Carcinogens
There are several different types of carcinogens. They include pathogens, radiation, and chemicals. Some carcinogens occur naturally. Others are produced by human actions.
- Viruses cause about 15 percent of all human cancers. For example, the virus called hepatitis B causes liver cancer.
- UV radiation is the leading cause of skin cancer. The radioactive gas known as radon causes lung cancer.
- Tobacco smoke contains dozens of carcinogens, including nicotine and formaldehyde. Exposure to tobacco smoke is the leading cause of lung cancer.
- Some chemicals that were previously added to foods, such as certain dyes, are now known to cause cancer. Cooking foods at very high temperatures also causes carcinogens to form (see Figure below).
Barbecued foods are cooked at very high temperatures. This may cause carcinogens to form.
How Cancer Occurs
Mutations that lead to cancer usually occur in genes that control the cell cycle. These include tumor-suppressor genes and proto-oncogenes.
- Tumor-suppressor genes normally prevent cells with damaged DNA from dividing. Mutations in these genes prevent them from functioning normally. As a result, cells with damaged DNA are allowed to divide.
- Proto-oncogenes normally help control cell division. Mutations in these genes turn them into oncogenes. Oncogenes promote the division of cells with damaged DNA.
Cells that divide uncontrollably may form a tumor, or abnormal mass of cells. Tumors may be benign or malignant. Benign tumors remain localized and generally do not harm health. Malignant tumors are cancerous. There are no limits to their growth, so they can invade and damage neighboring tissues. Cells from malignant tumors may also break away from the tumor and enter the bloodstream. They are carried to other parts of the body, where new tumors may form. The most common and the most deadly cancers for U.S. adults are listed in Table below.
Most Common Types of Cancer after Skin Cancer (% of all cancers)
Most Common Causes of Cancer Deaths (% of all cancer deaths)
prostate cancer (33%), lung cancer (13%)
lung cancer (31%), prostate cancer (10%)
breast cancer (32%), lung cancer (12%)
lung cancer (27%), breast cancer (15%)
More cancer deaths in adult males and females are due to lung cancer than any other type of cancer. Lung cancer is most often caused by exposure to tobacco smoke. What might explain why lung cancer causes the most cancer deaths when it isn’t the most common type of cancer?
Cancer Treatment and Prevention
Most cancers can be treated, and some can be cured. The general goal of treatment is to remove the tumor without damaging other cells. A cancer patient is typically treated in more than one way. Possible treatments include surgery, drugs (chemotherapy), and radiation. Early diagnosis and treatment of cancer lead to the best chance for survival. That’s why it’s important to know the following warning signs of cancer:
- change in bowel or bladder habits
- sore that does not heal
- unusual bleeding or discharge
- lump in the breast or elsewhere
- chronic indigestion or difficulty swallowing
- obvious changes in a wart or mole
- persistent coughing or hoarseness
Having one or more warning signs does not mean you have cancer, but you should see a doctor to be sure. Getting routine tests for particular cancers can also help detect cancers early, when chances of a cure are greatest. For example, getting the skin checked regularly by a dermatologist is important for early detection of skin cancer (see Figure below).
Regular checkups with a dermatologist can detect skin cancers early. Why is early detection important?
You can take steps to reduce your own risk of cancer. For example, you can avoid exposure to carcinogens such as tobacco smoke and UV light. You can also follow a healthy lifestyle. Being active, eating a low-fat diet, and maintaining a normal weight can help reduce your risk of cancer.
Air Pollution and Illness
Almost 5 million people die each year because of air pollution. In fact, polluted air causes more deaths than traffic accidents. Air pollution harms the respiratory and circulatory systems. Both outdoor and indoor air can be polluted.
Outdoor Air Pollution
The Air Quality Index (AQI) is an assessment of the pollutants in the outdoor air based on their human health effects. The health risks associated with different values of AQI are shown in Figure below. When AQI is high, you should limit the time you spend outdoors. Avoiding exposure to air pollution can help limit its impact on your health. People with certain health problems, including asthma, are very sensitive to the effects of air pollution. They need to be especially careful to avoid it.
Air quality is especially important for sensitive people. They include people with asthma, other respiratory illnesses, and cardiovascular diseases.
AQI generally refers to the levels of ground-level ozone and particulates. Ozone is a gas that forms close to the ground when air pollutants are heated by sunlight. It is one of the main components of smog (see Figure below). Smog also contains particulates. Particulates are tiny particles of solids or liquids suspended in the air. They are produced mainly by the burning of fossil fuels. The particles settle in airways and the lungs, where they cause damage.
Smog clouds the city of Los Angeles, California. Visible air pollution in the form of smog is a sign that the air is unhealthy.
Indoor Air Pollution
Indoor air may be even more polluted than outdoor air. It may contain harmful substances such as mold, bacteria, and radon. It may also contain carbon monoxide. Carbon monoxide is a gas produced by furnaces and other devices that burn fuel. If it is inhaled, it replaces oxygen in the blood and quickly leads to death. Carbon monoxide is colorless and odorless, but it can be detected with a carbon monoxide detector like the one in Figure below.
A carbon monoxide detector warns you if the level of the gas is too high.
Bioterrorism is the intentional release or spread of agents of disease. The agents may be viruses, bacteria, or toxins produced by bacteria. The agents may spread through the air, food, or water; or they may come into direct contact with the skin. Two of the best known bioterrorism incidents in the U.S. occurred early in this century:
- In 2001, letters containing anthrax spores were mailed to several news offices and two U.S. Senate offices. A total of 22 people were infected, and 5 of them died of anthrax.
- In 2003, a deadly toxin called ricin was detected in a letter addressed to the White House. The letter was intercepted at a mail-handling facility off White House grounds. Fortunately, the ricin toxin did not cause any illnesses or deaths.
- A carcinogen is anything that causes cancer. Most carcinogens produce mutations in genes that control the cell cycle.
- Both outdoor and indoor air may contain pollutants that can cause human illness and death.
- Bioterrorism is the intentional release or spread of agents of disease.
1. What is a carcinogen?
2. How do most carcinogens cause cancer?
3. Identify three ways cancer can be treated.
4. List four warning signs of cancer.
5. Define bioterrorism.
6. How can you use the Air Quality Index to protect your health?
7. Explain why ground-level ozone is usually a worse problem in the summer than in the winter in North America.
8. Compare and contrast pollutants in outdoor and indoor air, including their effects on human health.
Points to Consider
High levels of certain hormones can increase the risk of some types of cancer. For example, high levels of estrogen can increase the risk of breast cancer. Estrogen is a sex hormone.
- What are sex hormones? How do sex hormones normally affect the body?
- Do you think sex hormones might differ in males and females? Why?
Opening image courtesy of Bruce Wetzel/Harry Schaefer/National Cancer Institute (http://visualsonline.cancer.gov/details.cfm?imageid=1762), colorized by Sam McCabe, and is in the public domain.