Chemical categorization toxicity

A commonly used scheme for categorizing toxicity is based on exposure duration. Toxicologists generally seek to understand the effects of acute, chronic, and subchronic exposures. They attempt to learn for each of these three exposure categories the types of adverse effects a chemical produces, the minimum dose at which these effects are observable, and something about how these adverse effects change as the dose is increased. 

The schedule of chemicals (refer to Chapter 2 for the list) is found in the Annex on Chemicals of the CWC. The schedule of chemicals categorizes the CWC-regulated chemicals into three schedules (1, 2, and 3) according to the chemical properties, toxicity, application, and level of regulation by the CWC. The number of analytical data that is in the version of the OCAD released in October 2002 is presented in Table 1. 

An emergency response incident that involves the release of any chemicals or toxic materials will typically be categorized as a hazardous materials (Hazmat) incident. The response to a Hazmat incident is somewhat standardized across the country. Specialized Hazmat teams are normally called in to address these situations. Hazmat teams are typically part of the fire services and will possess a majority of the locality s chemical... 

Approaches to categorizing toxic chemicals include chemical class based on molecular structure the pathway by which exposure takes place the source or origin of a toxin the context in which a chemical is used and the mechanism by which a chemical exerts its toxic effect at the molecular level. Any chemical can be assigned to more than one category, making classification schemes partial, at best. 

For starters, if you manufacture or import chemicals then the registration process defined under TSCA must be completed. If you use chemicals categorized as toxic then your process should not endanger the environment or your workforce. You are responsible if these chemicals are inadvertently released into the environment in any way or misused by you. In addition, you are responsible for the final disposal of the chemical and any waste products. This is to say that you must plan ahead on how to safely use the chemical, properly clean up spills or contamination, and properly dispose of the chemical. Some people call this a cradle-to-grave procedure or process. 

The recommended treatment option for control of toxic pollutants regulated under PSES categorical standards is chemical substitution. Although chemical substitution of sodium hydrosulfite for zinc hydrosulfite is recommended for control of zinc at groundwood mills, PSES for zinc were calculated using treatment performance data for lime precipitation.39... 

This type of categorization, while convenient, might be misleading. It perhaps suggests that all chemicals having a common target produce the same type of toxic effect on that target. This is not the case, and we shall reveal several examples in the next chapter. 

There are several possible ways to categorize chemical toxicity. Perhaps the most common is by grouping chemicals according to the targets they can damage, and it is the approach followed here. 

The characteristics of a chemical or substance that would categorize it as a reactive material include (1) it reacts violently with water, (2) it forms potentially explosive mixtures with water, or (3) when mixed widi water or other chemicals, it generates toxic gases, vapors or fumes in a quantity sufficient to present a danger to human health or the environment. Because water is the most common fire suppressant, the characteristic of reactivity is especially relevant since the application of water to eliminate or prevent the spread fires may be counterpro-ductive rather than helpful. Several categories of chemicals will be discussed from this standpoint however, several of these same chemicals also present additional hazards. 

Given the enormous number of toxicants, it is difficult to classify them chemically, either by function or by mode of action, since many of them would fall into several classes. Some are natural products, many are synthetic organic chemicals of use to society, while others are by-products of industrial processes and waste disposal. It is useful, however, to categorize them according to the expected routes of exposure or according to their uses. 

Different enzymatic reactions that occur during the metabolism of chemicals have been well categorized. The biotransformation and conjugation reactions may lead to either the detoxication of the toxicant and the excretion of its metabolites, or to the activation of the toxicant into more reactive intermediates. These may, in turn, react with glutathione, or tissue proteins, or nucleic acids (e.g., RNA, DNA) and undergo different metabolic reactions. 

The Canadian new chemicals program uses two chemical inventories, the Domestic Substances List (DSL) and the Non-Domestic Substances List (NDSL). The DSL includes substances that were, between January 1, 1984, and December 31, 1986, in Canadian commerce, used for manufacturing purposes, or manufactured in or imported into Canada.17 It contains about 23,000 substances. One of the initiatives in the Canadian Environmental Protection Act, 1999 (CEPA, 1999) requires the Minister of the Environment and the Minister of Health to categorize (Section 73, CEPA 1999) and then if necessary, conduct screening assessments (Section 74, CEPA 1999) of substances listed on the DSL to determine whether they are toxic or capable of becoming toxic as defined in the Act. The NDSL is a list of substances not on DSL but in commerce elsewhere in the world.18 The NDSL contains more than 58,000 entries. 

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