Skin cancer, from human exposure

The most promising alternatives were synthetic pyrethroids. The pyrethroids, developed as derivatives of naturally occurring pyrethrum, cause hyperexcitation, aggressiveness, incoordination, whole-body tremor, and seizures. Acute exposure in humans, usually resulting from skin exposure due to poor handling procedures, usually resolves within 24 hours. While not particularly toxic to mammals, they can cause an allergic skin response in humans. Some pyrethroids may cause cancer, reproductive or developmental effects, or endocrine system effects. 

In general, the levels of arsenic in air and water are low, and the major source of human exposure is food. In certain parts of Taiwan and South America, however, the water contains high levels of this metalloid, and the inhabitants often suffer from dermal hyperkeratosis and hyperpigmentation. Higher levels of exposure result in a more serious condition gangrene of the lower extremities or blackfoot disease. Cancer of the skin also occurs in these areas. 

Paraquat, which was registered for use in 1965, is the most used of the bipyridilium herbicides. With a toxicity rating of five, it is reputed to have been responsible for hundreds of human deaths. 11 Exposure to fatal or dangerous levels of paraquat can occur by all pathways, including inhalation of spray, skin contact, ingestion, and even suicidal hypodermic injections. Chronic health effects from long-term exposure are reputed to include pulmonary effects, skin cancer, and Parkinson s disease.12 Despite these possibilities and its widespread application, paraquat is used safely without ill effects when proper procedures are followed. 

In 1993, a scientific link was established between ozone depletion and increases in ultraviolet radiation. It was found that increased exposure to UV-B radiation causes skin cancer, the formation of cataracts, and the suppression of the human immune system. Research has shown skin cancer to be as common as all other types of cancer combined. Sunscreens can protect humans from the risk of some skin cancers. Unfortunately they do not appear to provide protection against damage to the immune system. 

The toxicity, health effects, and related symptoms of poisoning caused by different metals and metal compounds in humans is modulated by many factors. In a large number of instances, poisoning from metal compounds is because of the persistence of the metal dusts and fumes present in the workplace, as well as the properties of each metal, the pattern or route of exposure, the form and nature of the metal, and the quantity or concentration of the metal compound ingested, inhaled, or absorbed into the system. The health status of a worker modulates its toxicity. Toxic metals cause severe poisoning and skin diseases such as melanosis, leukomelanosis, keratosis, nonpitting edema, gangrene, and skin cancer. 

The discovery that exposure to exogenous chemicals could lead to cancer in humans was first made in the late 18th century, when Percival Pott demonstrated the relationship between cancer of the scrotum and the occupation of chimney sweepers exposed to coal tar/soot. Other examples noted later were scrotal cancers in cotton spinners exposed to unrefined mineral oils, and cancers of the urinary bladder in men who worked in textile dye and rubber industries due to their exposure to certain aromatic amines used as antioxidants. Experimental induction of cancer by chemicals was first reported in detail by Yamagiwa and Ichikawa in 1918, when repeated application of coal tar to the ear of rabbits resulted in skin carcinomas. Over the next few years, Kennaway and Leitch confirmed this finding and demonstrated similar effects in mice and rabbits from the application of soot extracts, other types of tar (e.g., acetylene or isoprene), and some heated mineral oils. These researchers also observed skin irritation sometimes accompanied by ulcers at the site of application of the test material. Irritation was thought to be an important factor in skin tumor development. However, not all irritants (e.g., acridine) induced skin cancer in mice and conversely, some purified chemicals isolated from these crude materials... 

The potential for unusual health effects of chemical mixtures due to the interaction of chemicals or their metabolites (e.g., metabolites of trichloroethylene and benzene) in or with the biosystem constitutes a real issue in the public health arena. However, toxicity testing to predict effects on humans has traditionally studied one chemical at a time for various reasons convenient to handle, physiochemical properties readily defined, dosage could easily be controlled, biologic fate could easily be measured, and relevant data were often available from human occupational exposures. Chemicals are known to cause disease for example, arsenic and skin cancer, asbestos and lung cancer, lead and decrements of IQ, and hepatitis B predisposes to aflatoxin-induced liver cancer but the link between the extent of human exposure to even well-defined chemical mixtures and disease formation remains relatively unexplored, but of paramount importance to public health. 

Both chronic human studies and animal studies proved the causal relationship between cumulative UVR exposure and skin cancer (277), particularly non-melanoma skin cancer (NMSC). The link between the two is also evident from the fact that NMSC is most common on the head, neck, arms, and hands (278, 279). In particular, the correlation between UVR exposure and SCC seems to be very strong. Cutaneous SCC of the head and neck occurs almost exclusively on areas receiving maximal exposure (269,280). The link between BCC and cumulative exposure to UVR is not as evident (270, 281). Although BCC occurs on the face, head, and neck, unlike SCC, its distribution does not correspond well with the areas that receive maximum sun exposure (269). Case-control studies indicated that cumulative sun exposure is the most important risk factor for SCC, whereas inability to tan was the most important risk factor for BCC (270,282,283).Subsequently, it has been suggested that, for BSC, intermittent sun exposure, particularly in the childhood, may be more important than cumulative exposure (282). 

The layer of ozone that forms at an altitude of about 100,000 kilometers helps protest us from the Sun s short wave length ultraviolet radiation. Exposure to this radiation has been shown to increase the incidence of skin cancer. This ozone is chemically the same is the ozone discussed in this chapter but its production by sunlight and its value to humanity is entirely unrelated to the use of ozone described below. 

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