Chemicals, fumes, viruses, bacteria, low-level radiation, and other potentially harmful substances are common in the environment. When these substances are released in large quantities or get out of our control, they can become immediate public health threats. Guidelines for how to prepare for and avoid a problem often depend on how the particular substance is spread.

In general, a health threat may spread through a community:

• Through the air.
• Through the water supply or through food.
• From human to human.
• From animal or insect to human.

Air contamination

Chemicals are the most likely source of air contamination. An accident at a plant or factory or a train wreck might release large amounts of a hazardous chemical into the air, for instance. If bacteria or viruses causing diseases such as anthrax, pneumonic plague, smallpox, or tularemia were released in a spray (aerosol) form, anyone who inhaled the substance could be affected. While air itself does not become radioactive, release of radiation into the environment can create radioactive dust and dirt (fallout) that can make the air unsafe.

What to do

If a hazardous substance is released into the environment:

• Tune in to a local radio or TV station for instructions from public health and emergency officials. (Phone lines are likely to be overwhelmed during a public health emergency, so do not try to call for instructions.) Depending on the kind of release, authorities may advise you to shelter in place, or simply to stay indoors. You do not need to leave your community unless local authorities tell you to.
• Get out of the immediate area if possible. If the release has occurred outdoors, go inside. If it has occurred indoors, go outside. Move out of low-lying spots to higher ground-most chemicals released into the environment are heavier than air and will sink.
• If you are directly exposed to radioactive dust, dirt, or other fallout, follow the steps for personal decontamination.
• Do not take potassium iodide (KI) pills unless local authorities tell you to. These pills are effective against radioactive iodine only, and they can be harmful if taken improperly.

Water and food contamination

Chemicals, heavy metals like lead and mercury, and living organisms such as bacteria and viruses can all be threats to a safe water supply. These substances can also contaminate food. Unintentional contamination of water as a result of chemical leaks or spills, natural disasters, and other causes has been a much bigger problem than deliberate contamination. Likewise, accidental food contamination by botulinum toxin (the agent that causes botulism), E. coli, and other harmful organisms during the storage or preparation of food is much more likely than intentional food poisoning.

How to prepare

With the exception of a known accident (such as a chemical spill into the water supply), you probably would not know you had consumed contaminated water or food unless you developed symptoms.

To reduce your risk of consuming contaminated food or water and to be better prepared for public health emergencies affecting the water supply:

• Don't eat food or drink water or any other beverage that looks or smells suspicious. In general, it's not a good idea to eat or drink something when you don't know who has prepared or provided it or where it has come from.
• When shopping, avoid food or beverage items that look like they may have been tampered with-for instance, if the seal is broken or you suspect the container may have been opened.
• Remember that most cases of foodborne illness (food poisoning) happen by accident. For more information, see the topic Foodborne Illness and Safe Food Handling.
• Know generally where your household water comes from. Is it from the city water supply? Most public water supplies are carefully monitored and treated to guard against contamination. Does a private well supply your water? Private water supplies are unlikely to be targets of intentional contamination, but they can become contaminated accidentally and may not be as closely monitored as city water supplies.
• Consider storing emergency water and food supplies.
• Learn how to purify water, and make sure you include the supplies you will need in your emergency kit. Knowing how to purify water is useful in any situation where you have to rely on untreated water.

What to do

If there is an emergency affecting the water supply:

• Follow all instructions from local authorities about purifying your water (commonly called "boil orders") or using other water sources. Authorities will notify your community when it is safe to drink from the regular water supply again.
• Do not strictly ration emergency drinking water supplies. Try not to waste any water, but drink what you need. On average a person needs about 2 litres of water a day. (Individual water needs vary depending on age, health, diet, and climate.) Learn the signs of dehydration in children and adults so you know what to watch for.
• Use the safest water you have first before turning to other water sources.
• If you know or suspect that your skin has come in direct contact with water that has been contaminated by a hazardous chemical or radiation fallout, follow steps for personal decontamination.

Some bacteria, viruses, and other biological agents can be spread from person to person, or from animals or insects to people. The ease of international travel has made many of these health threats more difficult to contain. Recent health threats such as H5N1 influenza (avian flu), SARS (severe acute respiratory syndrome), the West Nile virus, and monkeypox have made people more aware of how easily disease can spread, not just within a community but from one community to the next.

The World Health Organization has current, reliable information about communicable diseases and health concerns throughout the world. Visit the agency's website at www.who.int/en for updates on specific health emergencies.

What to do

To reduce your chances of being infected with or spreading a contagious disease:

• Wash your hands with soap and water frequently.
• Do not share bedding, towels, utensils, or other items with someone who is sick. If you are sick, do not share these items with anyone else.
• Avoid exposure to disease-carrying animals and insects if you are in an area where these are a problem. Use insect repellents to reduce insect bites.
• Follow the advice of local health authorities if there has been a disease outbreak in your community or in an area where you are travelling. It is especially important to follow health experts' instructions if you live or work with someone who becomes sick. For instance, you may be advised to wear a properly fitted surgical mask if you are in close contact with someone who has a serious contagious illness.
• If there is an outbreak of a contagious disease in your area, do not leave the area unless authorities tell you to. If you have already been infected, you could spread the disease. Leaving the area may also cause a delay in your diagnosis or treatment.