Environmental Toxicology

STOP - You must read Chapter 9 and 17 before doing this lesson

Introduction

What is toxicology? Is the study of the poisons around us. Paracelsus in the sixteen century said that all substances are poisonous, what make the difference between a poison or a therapeutic agent is the dose.

Infectious Organisms

For most people in the world the greatest environmental health thread continues to be pathogenic organisms. Nearly three quarters of all deaths in industrialized countries are due to cardiovascular diseases or cancer, but in the developing countries, infectious agents and parasitic diseases cause almost half of all deaths.

Malnutrition and infectious diseases create a vicious cycle. Poor nutrition makes people more susceptible to infection, and infections in turn often result in diarrhea and vomiting, that makes it more difficult to obtain, absorb, and retain food.

Emergent Diseases

An emergent disease is one never known before or one that has been absent for at least 20 years. At least 30 new infectious diseases have appeared in the past two decades while many well-known diseases have reappeared in more virulent, drug-resistant forms. Diseases such as the Ebola virus (shown at the right), the Hanta virus, drug-resistant tuberculosis and other are some of the emergent diseases. (The graph below shows number of new tuberculosis cases in the USA for 1996).

A number of factors contribute currently to the appearance and spread of contagious diseases. With a population of 6 billion, human densities are much higher, enabling germs to spread further and faster than even before. Expanding populations push into remote areas, hunting animals that once would have never seen humans, converting land to agriculture, and being exposed to exotic pathogens and parasites. Elimination of predators and habitat changes favor disease-carrying organisms such as mice, rats, cockroaches, and mosquitoes.

The outbreak of Hanta virus in the American Southwest in 1993 is an example of how environmental changes helps spread disease. In 1992, after being absent for nearly a century, a cholera epidemic swept down the west coast of South America. More than 1 million people were infected and at least 12,000 died from this severe diarrhea disease.

Toxic Chemicals

Chemical agents represent the biggest group of toxicological thread to environmental and human health. They can be divided into two broad categories: those that are hazardous and those that are toxic.

Hazardous- mean dangerous. Examples are flammables, explosives, irritants, acids, based or alkaline substances. A lot of chemicals that are hazardous at high concentration are relative harmlesss when dilute.

Toxins- are poisonous. They react with specific cellular components to kill cells. They often are harmful even at low concentrations.

Irritants- are corrosives strong acids and alkaline reagents that damage biological tissues on contact. Skin diseases caused by irritants such as dermatoses and dermatitis are the most common occupational diseases.

Respiratory fibrotic agents- irritants that damage the lungs causing scar tissue formation that lowers respiratory capacity. Examples of fibrotic lung diseases include silicosis, asbestosis, black lung diseases and other.

Asphyxiants- are chemicals that exclude oxygen or actively interfere with oxygen uptake and distribution. Examples of these chemicals are carbon monoxide, hydrogen cyanide and other.

Allergens- substances that activate the immune system and cause allergies. They may act as antigens. For example formaldehyde has been associate to cause "sick house" syndrome.

Neurotoxins- specifically attack nerve cells (neurons). The nervous system is so important in regulating body activities that disruption of its activities is especially fast-acting and devastating. Anesthetics substances such as chloroform depress the nervous system. Organochlorine pesticides such as DDT, dieldrin, aldrin, and heavy metals such as lead and mercury, disrupt ion transport across the neuron cell membrane affecting the initiation of the nerve impulse. Organophosphate pesticides such as malathion and parathion, and carbamates pesticides such as sevian cause an overstimulation of the nervous system by inhibiting acetylcholinesterase. The inhibition of acetylcholinesterase prevents the hydrolization of acetylcholine a neurotransmitter.

Mutagens- substances that damage or alter genetic material or DNA in cells. Ionizing radiation such as alpha and beta particles, and x-rays and gamma rays are physical agents that can cause mutations.

Teratogens- are substance that cause abnormalities to the fetus during pregnancy, primarily during the first trimester of pregnancy (embryonic growth and development period). Examples of teratogens are alcohol (ethyl alcohol), mercury (methyl mercury), thalidomide, and other substances or chemicals.

Carcinogens- are substances that induce or cause cancer. Substances such as benzene, cigarette smoke, cathecol, radon and other have been prove to be human carcinogens.

National Toxicology Program web site.

CDC - Emerging Diseases web site.

Emerging Infectious Diseases