Health hazard classes

Health hazards are effects of a product on the human body. They range from immediate effects, such as burns, poisoning and eye damage, to effects that develop after repeated exposure over months or years, such as allergies, organ damage and cancer. Because many chronic health effects are not noticed until it is too late, understanding health hazard classes is one of the most important parts of hazard communication. This lesson explains how health hazards are classified, the acute toxicity categories and how they are determined, skin and eye effects, sensitization, the CMR hazards (carcinogenicity, germ cell mutagenicity and reproductive toxicity), specific target organ toxicity, aspiration hazard, and hazards that are covered differently in each country.

How health hazards are classified

Classification of health hazards relies on evidence from human experience, animal studies, laboratory tests and, increasingly, computational and in vitro methods. For mixtures, classification often uses calculation methods based on the hazards and concentrations of the ingredients, or bridging principles that apply data from similar mixtures. Classification describes the inherent hazard of the product, its potential to cause harm, not the risk in a particular situation, which also depends on how much exposure occurs. A highly toxic chemical used in tiny amounts in a closed system may pose less risk than a less toxic one sprayed in a small room. Lesson 10 discusses exposure and risk.

The health hazard classes

Acute toxicity

Acute toxicity describes harmful effects after a single dose or short exposure. It is classified separately for three routes of exposure: oral (swallowing), dermal (skin contact) and inhalation (breathing). The categories are based on the median lethal dose (LD50) or concentration (LC50): the dose or concentration that killed half of test animals, or an equivalent estimate. Lower LD50 values mean greater toxicity.

To put these numbers in perspective, an oral LD50 of 5 mg/kg corresponds to a few drops for an adult, while 2,000 mg/kg corresponds to well over 100 grams. Products in Categories 1 to 3 carry the skull and crossbones; Category 4 products carry the exclamation mark. Hazard statements specify the route, such as "Fatal if inhaled" (Categories 1 and 2), "Toxic in contact with skin" (Category 3) or "Harmful if swallowed" (Category 4).

Classifying mixtures for acute toxicity

For a mixture, the acute toxicity estimate (ATE) can be calculated from the ATEs and concentrations of its ingredients using the additivity formula:

Here \(C_i\) is the concentration of ingredient \(i\) in percent, and \(\text{ATE}_i\) its acute toxicity estimate. Ingredients of unknown toxicity are handled according to additional rules.

Skin and eye effects

Skin corrosion means irreversible damage, such as visible necrosis through the skin, which occurs with strong acids and bases such as sulfuric acid, sodium hydroxide and concentrated ammonia. Corrosives carry the corrosion pictogram and the signal word Danger. Skin irritation means reversible damage, such as redness and swelling, and carries the exclamation mark and Warning. Similarly, serious eye damage means damage that is not fully reversible within 21 days, carrying the corrosion pictogram, while eye irritation is reversible and carries the exclamation mark. Eyes are especially vulnerable: alkalis in particular penetrate deeply and can cause blindness within minutes, which is why immediate, prolonged flushing with water is critical (Lesson 11). Products with pH at or below 2 or at or above 11.5 are generally considered corrosive unless test data show otherwise.

Sensitization

Sensitizers cause allergic reactions. After an initial exposure, or repeated exposures, the immune system becomes sensitized, and later exposures to even tiny amounts trigger a reaction. Respiratory sensitizers can cause occupational asthma, which may be permanent; examples include isocyanates used in spray foam, paints and adhesives, flour and grain dust, some enzymes, and certain wood dusts. Skin sensitizers cause allergic contact dermatitis; examples include epoxy resin components, some hair dyes, nickel, latex proteins and some preservatives. Once sensitized, a person may need to avoid the substance completely for the rest of their working life. Respiratory sensitizers carry the health hazard pictogram and Danger; skin sensitizers carry the exclamation mark and Warning. Preventing the first sensitization through controls and protective equipment is crucial.

Carcinogens, mutagens and reproductive toxicants

Carcinogenicity, germ cell mutagenicity and reproductive toxicity are often grouped as CMR hazards. Each has categories: Category 1A means known to have the effect in humans, based largely on human evidence; Category 1B means presumed to have the effect, largely based on animal evidence; Category 2 means suspected. Category 1 carries the signal word Danger, and Category 2 Warning, both with the health hazard pictogram. Examples of carcinogens include asbestos, crystalline silica, benzene, formaldehyde and diesel engine exhaust. Reproductive toxicants, such as lead, some solvents and some pharmaceuticals, may harm fertility or development; hazard statements such as "May damage fertility or the unborn child" indicate the effects. Reproductive toxicity also includes an additional category for effects on or via lactation, with the statement "May cause harm to breast-fed children" and no pictogram.

Specific target organ toxicity

Specific target organ toxicity (STOT) covers significant effects on particular organs that are not covered by other classes. STOT single exposure includes Category 1 and 2 effects such as damage to the nervous system, liver or kidneys after one exposure, and Category 3 for transient effects: respiratory tract irritation and narcotic effects such as drowsiness and dizziness, typical of many solvents. STOT repeated exposure covers organ damage from prolonged or repeated exposure, such as lung damage from some dusts, nerve damage from n-hexane, or liver damage from some solvents. Hazard statements name the organ where known, such as "Causes damage to the lungs through prolonged or repeated exposure by inhalation".

Aspiration hazard

Aspiration hazard applies to liquids with low viscosity, such as many hydrocarbon solvents, gasoline and some lamp oils, that can enter the lungs if swallowed and then vomited, or inhaled directly as liquid. Even small amounts in the lungs can cause severe chemical pneumonia and death. The hazard statement "May be fatal if swallowed and enters airways" carries the health hazard pictogram and Danger. This is why first aid instructions for such products say not to induce vomiting.

Hazards not otherwise classified and other OSHA rules

HazCom includes hazards not otherwise classified (HNOC), adverse effects that do not meet the criteria of any GHS class, which must be disclosed on the SDS but are not required on the label. Biological hazards such as bloodborne pathogens are covered by other OSHA standards, such as the Bloodborne Pathogens standard, rather than HazCom. Some GHS categories are not adopted by OSHA: acute toxicity Category 5, the environmental hazard classes (which may appear voluntarily on SDSs, with environmental information required in section 12 as non-mandatory content), and some other categories. Substance-specific standards for chemicals such as lead, cadmium, benzene and silica add requirements for exposure monitoring, medical surveillance and specific warning language.

Interpreting health hazard information

Health hazard classifications should prompt questions: How could I be exposed? By breathing vapor, dust or mist; by skin contact; by swallowing through contaminated hands; or through the eyes? Is the effect immediate or long-term? Are there people who may be more vulnerable, such as pregnant workers or those with asthma? What controls are needed? SDS section 11 gives toxicological details and section 8 gives exposure limits and protective measures. Workers who believe they have been harmed by an exposure, or who notice symptoms such as skin rashes, breathing problems or headaches that improve away from work, should report them so they can be investigated.

Common health hazards in everyday workplaces

Some health hazards are so common that it is worth recognizing them by name. Crystalline silica, released when cutting, grinding or drilling concrete, stone and brick, causes silicosis and lung cancer and is classified as a carcinogen. Isocyanates, found in spray foam insulation, some paints and adhesives, are among the leading causes of occupational asthma. Solvents such as toluene, xylene and acetone can cause dizziness and headaches at high concentrations and, with long-term exposure, nervous system effects. Formaldehyde, used in some resins, preservatives and laboratories, is a sensitizer and carcinogen. Diesel exhaust is a human carcinogen. Cleaning products often contain corrosives and respiratory irritants, and mixing them can release toxic gases. Wet work, such as frequent hand washing and contact with water-based fluids, together with detergents and disinfectants, is a major cause of occupational dermatitis.

How evidence of health effects is gathered

Health hazard classifications rest on several kinds of evidence. Epidemiology studies patterns of disease in groups of people, such as workers exposed to a substance compared with those who were not, and provides the strongest evidence for effects in humans. Toxicology studies effects in laboratory animals and in cells, which allows controlled testing but requires judgment about relevance to humans. Case reports describe individual incidents. International bodies such as the International Agency for Research on Cancer (IARC) review evidence and publish classifications, which suppliers consider when classifying products. Because evidence grows over time, classifications can change, which is one reason SDSs and labels must be updated when significant new data appear.

Vulnerable workers

Some workers may be more vulnerable to certain chemical hazards. Pregnant and breastfeeding workers may need protection from reproductive toxicants. Young workers may have less experience recognizing hazards. Workers with asthma or skin conditions may react more strongly to irritants and sensitizers. Workers who do not read the language of the labels well need information in a form they understand. Employers should consider these factors when assessing risks and planning controls, and workers can ask for information or accommodation without fear of reprisal.

Symptoms that should prompt action

Workers should know the warning signs of chemical exposure and report them promptly: eye, nose or throat irritation; coughing or wheezing; skin redness, itching or rash; headaches, dizziness, nausea or unusual tiredness during or after work; and symptoms that improve on weekends or holidays and return at work. Reporting allows the employer to investigate exposures, adjust controls and arrange medical assessment where needed. Early action can prevent a temporary problem, such as mild irritation or early sensitization, from becoming a permanent condition.

Health hazard information on the SDS

Health hazard information appears in several SDS sections. Section 2 gives the classification and hazard statements; section 4 describes symptoms and first aid; section 8 gives exposure limits and protective measures; and section 11 gives the toxicological data, including routes of exposure, acute and chronic effects, and whether ingredients are classified as carcinogens, mutagens or reproductive toxicants. When a product has a serious health hazard, such as a carcinogen or respiratory sensitizer, reading section 11 helps you understand why the precautions matter and what symptoms to watch for.

Remember that the absence of a health hazard classification is not proof of safety: it may reflect a lack of data rather than evidence of no effect, so good hygiene and sensible controls apply to all products.