2. TOXICOLOGY of SOLID MATERIALS

 i. Pesticides

Pesticides are simply chemicals / materials designed to selectively kill unwanted organisms. Pesticides are typically categorized on the basis of the target organism(s). The table below indicates various pesticide groups and the targeted organisms.

Selected Pesticide Types

Since the earliest agrarian activities, people have attempted to improve yields and quality of the crops they harvested. Initial pest removal consisted of hand-picking weeds and other competitive growth from desired crops, but later civilizations employed chemicals such as arsenic-compounds (early Chinese, Romans) to reduce pests. Today, pesticides are used to help improve crop yields and enhance production by reducing a wide range of pests. Because of their extensive use in modern times, pesticides are a significant concern because humans are continually exposed to persistent traces/residues of these compounds through the foods they eat. The exposure may be direct (i.e. pesticides applied directly to food crops) or indirect (i.e. ingested by animals later consumed as food). One specific concern is the fact that pesticides (such as inorganic and organometallic compounds) are often quite toxic to humans at the levels needed to be effective against the target organisms. Because of this, both toxicology and persistence of pesticides (and other toxicants) must be closely examined.

One group of insecticides includes organochlorine compounds which have several characteristics. These compounds do not degrade easily (they are highly stable), they are soluble in fatty tissues, and have relatively low water solubility. These compounds also are relatively toxic to insects but less so to humans. An example of this type of typical organochlorine pesticide would be HCB (hexachlorobenzene). The combination of environmental persistence and affinity for fatty tissues make organochlorine compounds a continuing concern.

Another insecticide with both a high degree of notoriety and substantial health risks is DDT (para-dichlorodiphenyltrichloroethane). This compound was used extensively when it was first developed (during WW II) since it very effectively kills many disease-carrying insects and has a tremendous persistence in the area of application. DDT was used extensively to combat malaria (through the elimination of disease-carrying mosquito's) and to fight insect-borne diseases around the world.  It was the overuse of DDT, however, that led to dangerous environmental levels to the extent that bird mortality was greatly affected and avian fetal effects (soft shell formation, mutagenesis) were occurring.  In the 1960s when Canadian women showed elevated levels of DDT in breast milk there was great concern over the potential health effects. Currently, the use of DDT is now banned from almost all Western industrialized nations. This is slowly having a positive effect: the measured levels of DDT in breast milk of Canadian women had fallen to <10ng/g by 1992. In many developing nations however, heavy use of DDT continues, raising concerns over both health and safety. Two other examples of persistent organochlorine insecticides include toxaphene and  chlorinated cyclopentadienes.

Modern Insecticides include both organophosphate and carbamate compounds. While the organophosphates are far less persistent than their DDT predecessors, these compound are far more acutely toxic to humans. In particular, the people who apply and use these compounds are at far greater risk than the general population. On the positive side, these compounds decompose in a matter of days or weeks.   There are three main classes of organophosphates: Type A, (phosphates) such as "dichlorvos", Type B, (phosphorothioates) such as "parathion" and Type C (phosphorodithioates) such as "malathion". Despite a lack of persistence, non-target organisms (such as honey bees) can be killed by these compounds. The carbamates also have a low environmental persistence, as well as a low dermal toxicity. Examples include "Carbofuran" and "Carbaryl". "Aldicarb" is highly toxic to humans.

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Certain natural compounds are also used as insecticides, such as pyrethrins, extracted from chrysanthemum. Another example of a "natural pesticide" would include nicotine. These natural agents often have a toxicity approaching that of synthetic pesticides. A drawback to the use of natural compounds is that they are often unstable in sunlight. In general, a mixture of pest control strategies is the probably the most responsible and effective over the long term. Some of the methods could include biological control (using parasites for problem organisms), chemical treatments (appropriate natural/synthetic chemicals) crop rotation to minimize pest development, and regulation to avoid introduction of new pests.

ii. Herbicides

Agents which kill plant materials are termed "herbicides". They may affect a broad spectrum of plants or be quite specific for an individual problem weed. Today’s market is largely dominated by organic herbicides which are pest-specific and less likely to persist for extended periods.

Triazines are a modern herbicide that is based on an aromatic ring structure. An example of this type of herbicide is Atrazine, which has been employed since the 1950s to reduce grassy weeds in corn and other crops. This herbicide is degraded by microbes yielding non-toxic metabolites. There is a draw-back, however:  Atrazine is moderately soluble in water, and as a result can often be found in well-water (and other ground-water systems) in areas where its used. The removal of atrazine from potable water requires the use of PAC (powdered activated carbon) during filtration. While low concentrations of this agent do not appear to have harmful effects, high concentrations have been associated with both birth defects and cancer in humans. Atrazine is now listed as a possible carcinogen and its’ use has been banned from certain areas of the United States.

Other types of organic herbicides include chloroacetamides such as alachlor, (banned in Canada as a risk to health) and its' replacement, metolachlor, a similar compound. The EPA has recommended that the use of chloroacetamides be carefully regulated to avoid the risk of groundwater contamination.

Another group of herbicides includes the phenoxy compounds. Phenoxy herbicides have break-down products more toxic than the original molecule, degrading to produce phenol compounds. This group includes 2,4-D compounds used to kill broad-leaf weeds. Used widely on lawns and golf-courses, there are concerns over the development of cancer as a result of chronic over-exposure. The most toxic chemically-related compound is dioxin (a chlorophenol derivative), a suspect in increased incidences of cancer. These chloro-phenol types of compounds are also used as fungicides (wood preservatives).

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iii.   PCBs

Polychlorinated biphenyls (PCBs) are a serious environmental concern. They are stable, chemically inert liquids which are not readily flammable and are excellent electrical insulators. Because of this, these compounds were used extensively as coolants in transformers and other electrical equipment. Use of PCBs as a transformer coolant was halted in North America in the late 1970s, but there are still numerous transformers in operation that contain these compounds. When inadvertently released in to the atmosphere, PCBs persist for many years.  Low-level contamination of PCBs is world-wide, from the Great Lakes to the North and South Poles, and this contamination is expected to persist for many years due to the extremely slow decomposition of these compounds.  PCBs are also highly susceptible to bioaccumulation and bioconcentration in the organisms involved in many food chains. Concentrations in wildlife around affected watersheds are often thousands of times higher than that of the contaminated water itself. Although this remains a concern, the general trend has been an overall decrease in most organisms. For example, decreases have been measured at approx. 0.078% / year for PCBs measured in Lake Michigan. While PCBs do not have a high acute toxicity in humans, they are certainly a potential carcinogen in high doses and have suspected effects on fetal development.

PCBs (as well as dioxins and chlorinated furans) are not easily metabolized and tend to be stored in fatty tissues. Relative toxicity of these compounds can be expressed as TEQ (toxicity equivalency factor) which rates relative toxicity proportionally to the compound 2,3,7,8-TCDD (which is rated as being 1.0).

iv. PAHs

PAHs (polynuclear aromatic hydrocarbons) are multiple benzene structures which share carbon atoms between fused rings. These molecules are typically the product of combustion (i.e. soot, smoke) especially from industrial production of coal, etc.  These compounds are strongly suspected of having a serious impact on human health; not only as air-borne pollutants, but also as compounds leached into watersheds.

The most noted PAH is benzo-a -pyrene which is derived from burning / combustion of fossil fuels and is a concern due to both its carcinogenic nature and its high degree of bioaccumulation. The carcinogenic nature of BaP is not immediate: rather it is the biological breakdown of the compound in humans (via the cytochrome p450 system) which leads to the formation of carcinogenic compounds.

v. Heavy Metals

Heavy metals are extremely dense metals found naturally as trace elements. Some examples include mercury (Hg), Lead (Pb), Cadmium (Cd), etc. While some heavy metals are not highly toxic as elements, their cationic states are far more reactive. The degree of relative toxicity of a given heavy metal can therefore vary with the exact chemical form of the element. One particularly toxic form involves attachment of alkyl groups which permit passage through the blood-brain barrier. Heavy metals in these states are responsible for immediate, long-term and fetal toxic effects and can cause irreversible damage.

Bioaccumulation of heavy metals is similar to that observed in other toxic agents: the toxic agents become progressively more concentrated as they are consumed further and further up the food chain. Bioconcentration/accumulation is often a far more serious source of heavy metal exposure than through the direct route of drinking water. Since many heavy metals have a relatively long T_0.5 (half-life) (i.e. methyl mercury is 70 days) there is a long period for potential exposure and bioconcentration for humans consuming fish or other foods with elevated mercury levels. A summary of drinking water limits for heavy metals is listed below.

WHO Heavy Metal limits for Drinking Water

Mercury, (perhaps more than any other heavy metal) has achieved a high level of notoriety. Industrial production of certain chemicals (NaOH, Cl) often results in inadvertent / fugitive release of both gaseous and solid amalgams of mercury. Accumulation and bio-concentration of mercury in fish is a well-publicized concern in many water systems world-wide.  Mercury often forms methyl-mercury in aqueous systems, by reacting with light and various organic compounds. Methyl-mercury is of particular concern because of its affinity and persistence in fatty tissues and lipids in humans.

Mercury can also enter the ecosystem through leaching from soils/rocks and can be exasperated by activities such as dams (altering water levels) and flooding. Mercury poisoning is, and will continue to be a major issue in environmental chemistry for years to come.

Lead, a soft heavy metal, is now used far less in most western industrialized nations than previously. Previously used in leaded gasoline's, many paints and as "shot" for shotgun ammunition, these sources of lead contamination have been banned in Canada. Lead is still a component of certain solders used in electrical wiring and exists as a component of the plumbing in many older Canadian residences. The current major source of lead exposure for most Canadians continues to be drinking water. Long term accumulation of lead results in accumulation in the brain, often as a result of numerous years exposure and accumulation. Most vulnerable are fetuses and young children undergoing rapid brain growth/development.

Cadmium, is a by-product of industrial activities (such as zinc production) and is often found in high-power batteries in phones, portable tools and video cameras. When such batteries are incorrectly disposed of, each battery can release as much as several grams of cadmium, particularly a concern during incineration. Higher levels of cadmium can be found near smelters and mines, but the greatest general human exposure is through diet. Cadmium is absorbed by plants and is consumed by humans and animals. Acute toxicity is high (lethal dose < 1g) but cadmium itself is not biomagnified like mercury.

Arsenic, is commonly known as an acute poison but is introduced into the environment through the smelting / mining of metals (nickel/gold, etc.), through the burning of coal and as a component of pesticides. Considerable amounts of arsenic continue to be leached from abandoned gold mines resulting in considerable water contamination. Human intake is largely through drinking water, leading to carcinogenic toxicity, particularly in combination with smoking, although elevated skin cancer has also been associated with chronic exposure to arsenic.

REFERENCE - Canada's National Pollutant Inventory: http://www.ec.gc.ca/pdb/npri/npri_home_e.cfm