In order for environmental contaminants to cause cancers, or any other disease, there must be a completed pathway through which the contaminants could travel from their source, through the environment, to enter the human body through air, water, food, or direct contact with the skin. It is important that any environmental contamination that violates federal or state standards be rectified properly, whether or not such pollution is found to cause disease. Cancers today are usually related to events that happened many years ago. When a cancer is due to a contact with a cancer-causing agent, the disease does not develop immediately. Since the cancers we see now are generally related to a lifetime of certain habits or exposures to carcinogens, it is usually very difficult to pinpoint what caused a specific case of cancer.
Cancer clusters can occur by chance.
Cancer, and other diseases, do not occur evenly over time and place. The vast majority of occurrences of increased or decreased rates of cancer are due to random variation, even when unusually high or low rates can be statistically confirmed. What's more, when the numbers of cancer cases are small, it's particularly difficult for statistical analyses or scientific studies to yield useful or valid information. In the 1970s, when state cancer registries were first being organized, many public health scientists and others hoped that observations of clusters of cancer in the community might lead to the discovery of specific causes of these cancers. Since then, thousands of statistical analyses and countless intensive studies have taken place throughout the U.S., mainly conducted by state, local, or federal agencies.