TOXIC AND CARCINOGENIC CHEMICALS

"What is it that is not poison? All things are poison and nothing is without poison.
It is the dose only that makes a thing not a poison."
Paracelsus (1493-1541)

Toxicity Defined

The toxicity of a substance is due to its ability to damage or disrupt the metabolism of living tissue. An acutely toxic substance can cause damage as the result of a single or short-duration exposure. A chronically toxic substance causes damage after repeated or long-duration exposure or that becomes evident only after a long latency period. Carcinogens are considered a special class of chronic poisons.

It is understood that essentially all chemicals, at some concentration, are toxic. This section will focus on those chemicals which can be classified as extreme toxics or severe poisons, and those which have been shown to be toxic if exposure is long-term, i.e., chronic exposures, with special attention for those that possess carcinogenic characteristics. There are special problems associated with working with these compounds, and for regulated carcinogens in particular, additional precautions are prescribed by legislation. Topics considered here include identification of the hazardous materials, proper storage and handling techniques, and legal requirements.

Toxicology is the science that investigates the adverse reaction of chemicals on the biological system. The toxicity of a chemical, as defined in The Dose Makes the Poison, is related to its ability to damage an organ system, or to disrupt a biochemical process (such as the blood forming mechanism), or to disturb an enzyme system at some site in the body removed from the site of contact (as opposed to the affecting the site of contact as when a corrosive comes in contact with the skin). The systemic damage that a chemical does is not random - it affects the same set of body functions in all people. The sensitivity of individuals will vary and the effect may appear worse in some persons than others, but the target function or organ does not vary.

Poison Defined: Acute versus Chronic

Poisons are chemicals which cause illness, injury or death when taken in very small quantities. The legal definition of a poison is a chemical that takes less than 50mg per kilogram of body weight to kill 50% of the victims exposed. This is really a very small amount of material - about 3/4 of a teaspoon for the average adult and about 1/8 a teaspoon for a 2 year old child. There are very few chemicals that are lethal at these doses, but those that are must be classified as poisons. These materials will be classified as "acute poisons" because their effect is immediate.

Chronic toxicity, on the other hand, refers to the systemic damage that is done after repeated exposure of low concentrations over long periods of time. Materials most often associated with chronic toxicity are those that have been labeled as carcinogens, though there are other classes of chronic toxins which must be used with equal care. All chronically toxic materials are problematic because we do not know when or if the effect of the exposure will be felt. Workers in research laboratories and in other chemical settings should not discount any chemical exposure - materials not thought to be hazardous in the recent past are often found to be carcinogenic at a later time.

Most chemicals exhibit some degree of both acute toxicity and chronic toxicity. The symptoms displayed and the systemic effect will, however, differ. In addition, some materials may act as acute toxins, but show no chronic ill effects. The same is true for materials labeled as chronically toxic, which have no adverse single dose effect. Despite this lack of correlation, the effects of both forms of toxicity are definitely dose related, that is, the greater the dose, the greater the effect.

Examples and more detailed discussion of acute and chronic (carcinogenic and non) toxics follows:

• Acute Poisons
  • hydrogen cyanide
  • hydrogen sulfide
  • nitrogen dioxide
• Chronic Poisons
  • mercury
  • lead
  • vinyl chloride
• Carcinogens
  • benzene
  • carbon tetrachloride
  • cadmium compounds

Factors that Affect Toxicity

 

1. Routes of exposure

Toxicity varies with the route of exposure and the effectiveness at which the material is absorbed. A chemical that enters the body in large quantities but is not easily absorbed is a much lower risk than one that is easily absorbed into the bloodstream.

* skin contact

Perhaps the most common route of exposure is through skin contact. Fortunately the skin acts as an effective barrier against entry by most chemicals and thus greatly reduces the possibility of a toxic exposure. This is not true, however, if the skin is not intact, i.e., if there is an open cut. General rule: most inorganic chemicals are not easily absorbed through the skin, organic chemicals may or may not be absorbed, depending on numerous conditions. Some chemicals, such as DMSO (dimethyl sulfoxide) greatly enhance absorption of other chemicals through the skin, so particular care should be used with these materials. Once a chemical passes through the skin it enters the bloodstream and is carried to all parts of the body.

* inhalation

This is the most dangerous route of entry into body because the lungs are not an effective barrier to entry. The lung membrane allows ready passage of gases necessary to sustain life (a good thing!), but sadly they just as readily allow passage of chemicals that can be fatal (not a good thing!) Chemicals that pass the lung membrane are absorbed into the bloodstream and carried to all parts of the body. Absorption can be extremely rapid. The rate of absorption depends on the concentration of the toxic substance, its solubility in water, the depth of respiration and the rate of blood circulation.

* ingestion

Ingestion of toxic materials is an unlikely event in the chemical laboratory as long as good hygiene practices are followed. Materials that are ingested may be absorbed into the bloodstream anywhere along the gastrointestinal tract. If the material cannot be absorbed it will be eliminated from the body.

2. Species

Toxicity is species specific, with the level of sensitivity dependent upon each species. The difference in observed reaction is related to the method that each species handles the substance. The rate that the chemical is absorbed, metabolized or excreted in a greater or lesser amount, or the metabolic pathway that is utilized to handle the material will determine the end effect. Occasionally a physiological difference will determine the fate of the toxicant. For instance, some animals are not capable of vomiting so when they ingest a poison they have no means of removing the offending substance from the body.

3. Gender

Though the data for humans is almost nonexistent, there are definitive results that show there are gender differences in the sensitivity to certain chemicals. These differences are often not evident in the immature animals, indicating that the sensitivity is perhaps related to sex hormone production. On the other hand, there is ample evidence to show that the gender specific reproductive progress can be severely compromised by exposure to toxic substances.

4. Health

Individuals that are predisposed to certain health problems, such as diseases of the liver or lungs, are more likely to be affected by exposure to toxic materials, and once exposed, more likely to experience a more severe reaction.

5. Presence of other substances

Certain substances are dangerous when a person is exposed to 2 or more substances at the same time. The resulting effect is more hazardous than would be predicted from the exposure to either of the individual substances. This is know as a synergistic effect.

Identifying Toxic Materials

1. Using Chemical Structure as a Guide to Toxicity and Carcinogenicity

Unfortunately, it is not often easy to predict which class of chemicals are going to be toxic or carcinogenic, and which are not. There are some generalizations, however, that are possible. The following classes of chemicals have been found to be acute and chronic toxins, and extra care should be taken when working with them.

Acute toxins

• all halogens are toxic (bromine, chlorine, fluorine, iodine).
• cyanides and nitriles (CN groups) are rapid acting toxins.
• heavy metals (arsenic, cadmium, mercury, etc.) are well known toxins, some acute, others chronic.

Chronic toxins

• heavy metals (arsenic, cadmium, mercury, etc.) are well known toxins, some acute, others chronic.

Carcinogens

• alkylating agents (alpha-halo ethers; sulfonates; epoxides; electrophilic alkenes and alkynes)
• acylating agents
• organohalogen compounds
• hydrazines
• N-nitroso compounds
• aromatic amines
• aromatic hydrocarbons
• many "natural products"

Specific examples of these agents (acute toxins, chronic toxins and carcinogens) are also available.

2. Using Chemical Labels as an Aid

As an aid in identifying the chemicals which pose a reactivity hazard in the laboratory, all chemical manufacturers are required to include relevant information on the chemical label. One of the most common grading systems is that developed by the National Fire Protection Association (NFPA). In this system, chemicals are rated from 0 (non-toxic) to 4 (extremely toxic). It is important for all laboratory personnel to recognize and become familiar with the NFPA diamond and understand the grading levels established by the NFPA for toxic materials. The blue portion of the diamond gives an indication of the toxicity of the material.

NFPA System for Rating the Toxicity of Chemicals

Rating     Type of Possible Injury

  0	Materials that on exposure under fire conditions offer no hazard
	beyond that of ordinary combustible materials.
			
  1	Materials that on exposure would cause irritation but only 
	minor residual injury.

  2	Materials that on intense or continued but not chronic exposure 
	could cause temporary incapacitation or possible residual injury.

  3	Materials that on short exposure could cause serious temporary or
	residual injury.

  4	Materials that on very short exposure could cause death or major 
	residual injury. 

One difficulty with the NFPA labeling system is that it offers no indication as to whether the material is a carcinogen or potential carcinogen. You must consult the chemical label or material data safety sheet to obtain this information.

An even more obvious warning that a material is a poison is the presence of the familiar skull and crossbones on the chemical label. This is a standard symbol required by the Department of Transportation (DOT) to be used on all packages offered for transport over public highways, airways or by sea, which carry materials classified as an inhalation hazard, a poison or poison gas.

More recently, in an effort to warn small children of the dangers of toxic materials, a new picture has been introduced and widely used by the media and schools. Many substances, especially those that may be found in the home or school where children may be present, now carry the fluorescent green "Mr. Yuck" symbol prominently on their label.

And finally, the symbol which is now used to indicate that a material is a carcinogen or potential carcinogen is the cropped repeating "C".

It is important that all lab personnel are familiar with these symbols that are associated with toxic materials. Always look for these warnings! Never handle a potentially poisonous chemical until you are aware of the hazards, the level of protection required to work safely with the material, and the appropriate response should you be exposed to the substance. Some toxic materials will list an antidote on the label in case of accidental exposure. Always be aware of this antidote and be certain that it is within easy access of the area where the toxic material will be used.

3. Materials Recognized by Reporting Authorities to be Hazardous to Health

Three agencies are responsible for evaluating data on carcinogenicity or otherwise regulating the use of these materials. They are the International Agency for Research on Cancer (IARC), the National Toxicology Program (NTP), and the Occupational Safety and Health Administration (OSHA). These agencies each perform very different functions with regards to determining carcinogenicity, analyzing the results on this research, and making recommendations.

A. International Agency for Research on Cancer

The International Agency for Research on Cancer (IARC) was established in 1965 by the World Health Organization. IARC's mission is to coordinate and conduct research on the causes of human cancer, and to develop scientific strategies for cancer control. The Agency is involved in both epidemiological and laboratory research, and disseminates scientific information through meetings, publications, courses and fellowships.

Since 1969, the IARC has published 44 monographs considering the risk of cancer of various chemicals, mixtures and exposure circumstances, i.e. occupations. The IARC does not make recommendations regarding regulatory standards, but rather evaluates scientific studies. Materials that are studied are classified into one of 3 categories:

• Group 1: The material is carcinogenic to humans.
• Group 2A: The material is probably carcinogenic to humans.

  This category is used if there is limited evidence of carcinogenicity in humans and sufficient evidence in experimental animals. Or it can be placed in this category if there is sufficient evidence of carcinogenicity in humans or experimental animals, strengthened by other supporting evidence.

• Group 2B: The material is possibly carcinogenic to humans.

  This category is used when there is limited evidence of carcinogenicity in humans but no or inadequate supporting evidence in experimental animals. A material may also be placed in this group if there is no or inadequate evidence in humans, but limited evidence in experimental animals coupled with other supporting evidence.

B. National Toxicology Program

The NTP (National Toxicology Program) prepares the annual report on carcinogens which is issued by the Secretary of the Department of Health and Human Services. Carcinogenic substances are grouped as "known carcinogens" or "reasonably anticipated to be carcinogens". The NTP prepares annual reports on materials that have been shown or are suspected to cause cancer in humans.

List of chemicals known to be carcinogens

Many materials have been shown to cause cancer in humans; these materials are known to be carcinogens if there is sufficient evidence of carcinogenicity from studies in humans which indicates a causal relationship between the agent and human cancer.

List of chemicals reasonably anticipated to be carcinogens

The NTP designates a material as an anticipated carcinogen if:

1. There is limited evidence of carcinogenicity from studies in humans, which indicates that causal interpretation is credible, but that alternative explanations, such as chance, bias or confounding, could not adequately be excluded, or
2. There is sufficient evidence of carcinogenicity from studies in experimental animals which indicates that there is an increased incidence of malignant tumors: (a) in multiple species or strains, or (b) in multiple experiments (preferably with different routes of administration or using different dose levels), or (c) to an unusual degree with regard to incidence, site or type of tumor, or age at onset. Additional evidence may be provided by data concerning dose-response effects, as well as information on mutagenicity or chemical structure.

C. Occupational Safety and Health Administration

The Occupational Safety and Health Administration (OSHA) does not maintain a list of known carcinogens but does regulate a number of specific carcinogenic materials through standards. The standards provide very strict guidelines on handling, use and storage of these materials, including information regarding sampling, medical monitoring, training, labeling and hazard communications. A list of these OSHA regulated materials is available.

Exposure Limits

The greatest danger of overexposure to hazardous materials occurs through inhalation. Because of this, three agencies provide infromation regarding exposure limits, specifying levels of air borne contaminants which are considered safe. OSHA has established a list of 425 substances which are considered air contaminants, many of which are commonly found in the research lab. Each of these materials has associated with it a PEL or permissible exposure limit. In addition, there are two other authorities interested in measuring air contaminants - the National Institute of Occupational Safety (NIOSH) and the American Conference of Governmental Industrial Hygienists (ACGIH). These 3 agencies have established guidelines and specific exposure levels of various contaminants. The levels recommended by these agencies should be used as guidelines, and in addition, the PELs established by OSHA are considered legal limits with the power of the legal system behind them.

1. Terminology Related to Exposure Limits

The terminology used by each of these authorities (OSHA, NIOSH, ACGIH) is critical in assessing the risk of a hazardous materials. All lab personnel should take time to become acquainted with these terms and acronyms, which are commonly encountered in the Toxicological Information section of the material safety data sheet (MSDS), prior to handling a potentially hazardous material.

A. Occupational Safety and Health Administration (OSHA)

The first legislative action that responded to worker health issues was the Occupational Safety and Health Act of 1970, with enforcement authority granted to the newly created Occupational Safety and Health Administration (OSHA). With the chemical release disaster in Bhopal in 1984 which killed 2,000 people and injured 30,000, and heightened awareness of the public of the effects of contamiants in the environment, Congress began studying other plant disasters, leading to passage of the Right to Know Laws of 1986. This required that safe levels of specific chemicals be determined and exposure guidelines be set. The following terms are those used by OSHA with regards to exposure limits.

PEL (Permissible Exposure Limit)

the maximum allowable limit for an air contaminant for which a worker may be exposed on a daily basis without suffering adverse affects

C (Ceiling)

The concentration of a substance that should not be exceeded.

TWA (Time Weighted Average)

The airborne concentration of a material to which workers may not exceed for an eight hour day of a 40 hour week. This level may not be appropriate for the old, young, ill or those predisposed to problems from chemical exposures.

AL (Action Level)

The exposure level at which OSHA regulations for protective programs must be put into effect. This would include such things as air-monitoring, medical surveillance and training.

B. American Conference of Governmental Industrial Hygienists (ACGIH)

The American Conference of Governmental Industrial Hygienists is a not-for-profit organization which addresses the administrative and technical aspects of worker health and safety. ACGIH serves as a medium for the exchange of ideas and experiences to facilitate the promotion of standards, recommendations, and techniques in occupational and environmental hygiene. It offers support to the industrial hygiene profession in the anticipation, recognition, evaluation, and control of job-site hazards that may result in injury, illness, or well-being of workers. This organization has determined exposure limits for over 600 substances.

TLV (Threshold Limit Value)

The airborne concentration of a substance which nearly all workers may be exposed day after day without adverse effects.

TLV-TWA (Threshold Limit Value - Time Weighted Average)

The allowable 8-hour a day concentration that a worker may be exposed to during a 40-hour week.

TLV-C (Threshold Limit Value - Ceiling)

The ceiling value that should not be exceeded even for an instant. Unlike the other TLVs which serve as guidelines, the TLV-C must be viewed as an absolute boundary.

TLV-STEL (Threshold Limit Value - Short-Term Exposure Limit)

The short term exposure or maximum concentration of a substance which a worker may be exposed to for a continuous 15 minute period, with a low probability of experiencing irritation, irreversible damage, or unconsciousness. Four of these 15 minute periods are allowed per workday, with at least a 60 minute break in between. However, at no time may the TLV-TWA be exceeded.

C. National Institute of Occupational Safety (NIOSH)

REL (Recommended Exposure Level)

The highest airborne contamination level that a person may be exposed to and expect not to be injured. It may be expressed as a ceiling level or as a TWA (time weighted average) for a 10 hour work day.

IDLH (Immediately Dangerous to Life and Health)

This is the concentration above which is immediately dangerous. It is the value that is used in selecting an appropriate respirator.

2. Dose-Response Principles

The relationship between the dose and response can be represented graphically as seen in the accompanying graph. With initial exposure, no effect is seen. This can be interpreted as the range at which the body successfully mounts its own defense mechanisms and fights off the effects of the material. This remains true until the threshold is reached and surpassed. At that time, small incremental increases in the material results in comparatively larger responses. At this point the victim begins to display symptoms of exposure. As the dose continues to be increased, the maximum effect is reached at which point further increases in the material yield no additional change or response to the material. The ultimate maximum effect, of course, would be death of the victim.

The dose-response relationship will be highly dependent from one species to another, from one individual to another within a species, and perhaps even for the same individual given different testing conditions. So how can you set explicit levels that will guarantee that a given exposure will not cross the threshold? The FDA has done this by arbitrarily setting a 100-fold margin of safety. That is, if a test animal has a threshold of 100 ppm, the FDA has set the safe level for humans at 1 ppm. Why, you might ask, is the level set at 100-fold? This is based on the assumptions that humans are 10-times as sensitive to the material as animals, and that the weak portion (the old, young, ill, predisposed) of the population is 10-times as sensitive as the healthy human population.

It becomes apparent that the crucial value in determining the toxicity of a material is the threshold value. To determine this value, laboratory animals are used to establish the onset of symptomatic reactions. From these studies, toxicity data is gathered, threshold values are determined, and the results are reported. These are the values that are commonly reported in the material safety data sheets (MSDS) provided by chemical manufacturers.

3. Terminology Related to Toxicity Data

The following terms are ones that you will encounter on the material safety data sheet (MSDS) for all chemicals provided by a chemical manufacturer. As mentioned above in the section relating the dose to the response, many experiments have been done to determine the dangerous concentration levels of hundreds of materials. These results are tabulated and available in many references, as well as the MSDS, and it is important that all lab personnel are familiar with the degree of toxicity of the materials in use.

LC₅₀ (Lethal Concentration 50)

The concentration of a material in air that, on the basis of laboratory tests, is expected to kill 50% of a group of test animals when administered as a single respiratory exposure in a specific time period.

LC_LO (Lethal Concentration Low)

The lowest concentration of substance in air reported to have caused death in humans or animals. The reported concentrations may be entered for periods of exposure that are less than 24 hours (acute) or greater than 24 hours (subacute and chronic).

LD₅₀ (Lethal Dose 50)

The single dose of substance that causes the death of 50% of an animal population from exposure to a substance by any route other than inhalation.

LD_LO (Lethal Dose Low)

The lowest dose of a substance introduced by any route, other than inhalation, reported to have caused death in humans or animals.

TC_LO (Toxic Concentration Low)

The lowest concentration of substance in air to which humans or animals have been exposed for any given period of time that has produced any toxic effect in humans or produced tumorigenic or reproductive effect in animals or humans.

TD_LO (Toxic Dose Low)

The lowest dose of a substance introduced by any route other than inhalation over any given period of time and reported to produce any toxic effect in humans or to produce any tumorigenic or reproductive effect in humans or animals.

Precautions to Use When Working with Toxic Materials

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