Air and Water Purity

Humans are susceptible to contaminated air and water. Breathing in air that is laden with a noxious substance can cause illness or even death. Similarly, drinking water that contains an inorganic or organic poison, or an infectious microorganism can be debilitating or lethal.

Both water and air are particularly vulnerable to contamination by some bacteria and protozoa, and by their toxic products. While the contamination of air and water can be inadvertent, the noxious substances can also be introduced deliberately. Chemicals can also be dispersed in water and by air. A recent example occurred in 1995, when the Japanese cult Aum Shinrikyo released sarin gas into the Tokyo subway system. The poisonous gas attack killed 12 people and sickened 5,000.

As another example, in the months following September 11, 2001, there were several deliberate releases of anthrax spores into the air following the opening of contaminated letters. As well, the vulnerability of water supplies to contamination with a variety of infectious organisms has been recognized.

An amount as small as a glass of water can be contaminated with a quantity of organic or inorganic poison or microbe sufficient to cause harm. Even if the water has been chlorinated, disease causing microorganisms such as Giardia and Cryptosporidium are resistant to chlorine, as are bacterial toxins .

Technologies exist to kill the microorganisms that might be present (disinfection) or to completely remove the microbes and chemicals from the air or water (purification). These technologies, however, are usually designed to remove naturally occurring or polluting contaminants.

Groundwater or surface water treatment focuses on providing water that is fit to drink. Typically, the water is filtered to remove large debris. Some jurisdictions also pass the water through microfilters that remove objects as small as viruses from the treated water. Most drinking water is treated with chlorine or chlorine-containing compounds to kill any bacteria. Other treatments that are gaining widespread acceptance include the use of ultraviolet light, ozone, and other chemicals such as bromine. Water can also be purified by techniques involving reverse osmosis and steam distillation, although these techniques are not typically used, as they are expensive and purify relatively small volumes of water at one time.

Treatment and monitoring ensure that the water emerging from the treatment plant is safe to drink and that it remains that way all the way to the consumer's tap. However, these measures are not intended to thwart a deliberate contamination.

Yet for large surface water supplies, the volume of water alone makes the possibility of deliberate contamination remote. For example, it has been estimated that the contamination of the Crystal Springs Reservoir, which supplies some of the water for San Francisco, California, with enough hydrogen cyanide to harm anyone who drinks a glass of water would require over 400,000 metric tons of the poison. Similarly, huge amounts of bacteria or viruses would be required.

Air is vulnerable to contamination with a variety of bacteria, viruses, and fungi that are light enough to become dispersed in air currents. When inhaled, the microbes can cause infections. Chemicals and toxins can also float in the air, to be inhaled or settle onto exposed skin.

Air purification has long been possible using filters. Bacteria, viruses, and even some inorganic chemicals can be retained on specialized filters. These filters are mainly suitable for laboratories or relatively small, specifically designed ventilation systems. In large indoor environments such as malls or sizeable office buildings, and in the open air, air purification is virtually impossible.

Contamination of the open air poses a similar problem as the contamination of a large volume of water, namely the amount of poisonous agent that is required. For example, estimates are that hundreds of pounds of anthrax spores would be needed to achieve a massive contamination of the population of a large city.

The release of toxic agents into a more limited area such as an office building or a home is more plausible.

see also Air plume and chemical analysis; Bioterrorism; Organic compounds; Toxins.