While the approach to defining hazardous chemicals varies by regulatory agency, the symbols used to denote hazards is the same. The Globally Harmonized System (GHS) is a universally accepted system of labeling and communicating hazards. There are 29 GHS hazard classes that describe the hazard type and sometimes a chemical's current state (liquid, solid, or gas).

Hazard classes are broken down into three main types: physical hazards (17 classes), health hazards (10 classes), and environmental hazards (2 classes).

Since there are only 9 GHS pictograms for 29 classes, not every hazard class has its own symbol. Instead, the GHS pictograms represent multiple hazard classes with a similar type and level of risk.

Health Hazard Symbols and Meanings

There are four GHS symbols used to warn about health hazards like poisoning, tissue irritation, or causing disease.

All of the health hazard symbols mean you need to keep containers tightly closed until you've read more information and taken the recommended precautions.

Specific warnings and precautions will appear on the chemical label and in the SDS.

Toxic

The skull-and-crossbones GHS pictogram, widely known as the toxic symbol, represents chemicals with the highest levels of acute toxicity. That means these chemicals have an immediate and severe (even lethal) effect on human health.

Depending on the chemical, exposure may occur by inhaling, ingesting, or making skin contact with the substance. 

Examples include hydrofluoric acid, sodium azide and some powdered pigments and inks, which may contain toxic metals such as chromium and barium.

Harmful

The GHS pictogram with an exclamation mark is used on substances that are harmful or irritating. The health effects are acute (set in quickly), but they're less severe than something marked with the toxic symbol.

Chemical classes include:

• Skin, eye, or respiratory tract irritants
• Skin sensitizers, which cause an allergic response
• The lowest level of acutely toxic chemicals
• Materials with narcotic effects (drowsiness, lack of coordination, and dizziness)

Examples include acetone, o-Xylene and benzene.

Health Hazard

The health hazard pictogram is used for substances that present a health hazard over time.

Chemical classes include:

• Carcinogens, which cause cancer
• Respiratory sensitizers
• Teratogens, which are agents with reproductive toxicity that affects fertility or in utero development
• Chemicals with target organ toxicity
• Mutagenic chemicals that cause genetic defects
• Substances with aspiration toxicity

Examples include formaldehyde, mercury, and glyphosate - which is common in pesticides.

Physical Hazard Symbols and Meanings

There are five GHS symbols used to warn about physical hazards, which are characteristics that can put both property and human safety at risk.

Corrosive

The corrosive GHS pictogram represents substances that eat away at a material when they make contact. This symbol covers both physical and health hazard classes – that's why it depicts both a hand and a surface.

Chemical classes include:

• Skin corrosion or burns
• Eye damage
• Corrosive to metals

The GHS label will specify which materials or exposure routes can be corroded by a substance. Like all health hazard symbols, you should leave the container tightly sealed until you know more. Also, you'll need to consider the types of tools or containers you can safely use.

Examples include acetic acid, sodium hydroxide, photographic fixer and battery acid.

Explosive

The explosive GHS pictogram marks a chemical as unstable and capable of causing a fire or explosion.

Chemical classes include:

• Explosives
• Self-reactive chemicals
• Organic peroxides

The explosive pictogram means you need more information before you come anywhere near that container. Different chemicals can be set off under different conditions, including friction or mechanical shock – even bumping these containers with another object may be dangerous.

Examples include nitroglycerin, picric acid crystals, and benzoyl-peroxide initiators used in 3D printing.

Flammable 

The flammable GHS pictogram is used on chemicals that emit flammable gas or could self-ignite when exposed to water or air. 

Chemical classes include:

• Flammable gases, liquids, and solids
• Pyrophoric materials
• Self-reactive materials that aren't also explosive
• Organic peroxides that aren't also explosive
• Water-reactives

This pictogram means you should leave the container tightly closed and away from any sources of heat. You also need to be careful with storage conditions, including what other substances or materials get placed near these containers.

Examples include ethanol, gasoline, and mineral oils with <25 % DMSO (dimethyl sulfoxide) extract.

Oxidizing

The oxidizing GHS pictogram is used for oxidizing solids, liquids, and gases. These chemical classes, when exposed to oxygen, help ignite substances that wouldn't otherwise combust and/or make fires burn hotter and longer.

Like the flammable symbol, this pictogram also means you should leave the container tightly sealed until you know more. Don't change storage conditions or put other materials near these containers until you know what's safe.

Examples include compressed oxygen, hydrogen peroxide and sodium chlorate. 

Compressed Gas

The compressed gas GHS pictogram is used on gases that are stored under pressure.

Chemical classes include:

• Compressed gases
• Liquefied gases
• Refrigerated liquefied gases
• Dissolved gases

Containers with this hazard symbol may explode if heated. It's best not to even touch these containers until you know more because some require special handling precautions.

Examples include compressed oxygen, nitrogen, and hydrogen.

Environmental Hazard Symbol and Meaning

Only one of the GHS & OSHA pictograms addresses an environmental hazard.

The environmental hazard GHS pictogram indicates that a chemical is toxic to aquatic wildlife. This is the only GHS symbol that's not mandatory.

When you see this symbol, disposal of the substance may require a specific method. You also need to take precautions against release into the environment during use or transportation.

Examples include copper sulfate, silver biocides, zinc oxide pigments found in paints. 

Routes of entry describes the way in which a hazardous material enter the body. In a laboratory, the primary route of entry is through inhalation and dermal contact. With proper ventilation such as a chemical fume hoods and proper personal protective equipment, like nitrile gloves, you can reduce the risk of exposure to hazardous chemicals. Good hygiene habits, such as washing your hands after using hazardous chemicals, and using mechanical means to pick up sharps, will reduce your risk of exposure through ingestion or injection. Make sure to report any exposure, or possible exposure.

Absorption (Skin/Eye Contact)

Absorption may occur when not using proper engineering controls, work practices and procedures, and/or personal protective equipment (PPE). It may also occur when handling containers, etc. that have been contaminated without wearing PPE.

Reduce your risk of exposure through absorption by:

• Wearing appropriate gloves and other PPE
• Removing PPE when entering common spaces and hallways
• Clearly labeling areas that require PPE
• In the event of an accidental exposure, wash the affected area with soap and water for 15 minutes, and seek medical attention

Inhalation

Inhalation may occur when working with volatile materials without appropriate engineering controls, work practices, and/or PPE.

Reduce your risk of exposure through inhalation by:

• Using appropriate engineering controls, work practices, and protective gear to control for the inhalation hazards, for example a chemical fume hood or respirator.
• In the event of an inhalation exposure, seek medical attention for instructions

Ingestion

The gastrointestinal tract is another possible route of entry for hazardous materials. Although direct ingestion of laboratory substances is unlikely, exposure can occur as a result of ingesting contaminated food or beverages, touching the mouth with contaminated fingers or pens, or swallowing inhaled particles (indirect exposure).

Reduce your risk of exposure through ingestion by:

• Not eating, drinking, smoking, or storing food or drink in labs
• Washing hands thoroughly after working with hazardous materials, even when gloves are worn
• Washing your hands when leaving the lab and before eating, drinking, smoking or applying cosmetics
• In the event of accidental ingestion, seek medical care for instructions
• Do not induce vomiting unless directed to do so by a health care provider

Injection

Injection effectively bypasses the protection provided by intact skin and provides direct access to the bloodstream, and thus, to internal organ systems. Injection may occur through mishaps with needles, wires, when handling biting animals, or through accidents with broken glass or other sharp objects that have been contaminated.

Reduce your risk of exposure through injection by:

• Wearing appropriate gloves and other PPE
• Using caution when working with and around sharp objects
• In the event of an accidental injection, wash the affected area with soap and water for 15 minutes, and seek medical attention

The effects of exposure to a chemical is dependent on many factors:

• Dose
• Route of exposure 
• Physical properties of the chemical 
• The susceptibility of the individual receiving the dose. No two people are alike - each person's body will react differently upon exposure. Exposure to a hazardous material may affect one person more than others.

The toxic effects of hazardous materials may be local or systemic, acute or chronic.

• Local injuries involve the area of the body in contact with the hazardous material and are typically caused by reactive or corrosive chemicals, such as strong acids, alkalis, or oxidizing agents.
• Systemic injuring involve tissues or organs unrelated to or removed from the contact site when toxins have been transported through the bloodstream. Certain hazardous materials may affect a target organ.
• Acute toxicity results from a single, short exposure, and the effects usually appear quickly and may be reversible.
• Chronic toxicity results from repeated exposure over a long period of time. Effects are usually delayed and gradual, and may be irreversible.

Dose

The dose is the amount of a chemical that actually enters the body. This dose of a chemical that a person receives is dependent on the concentration of the chemical and the frequency and duration of exposure. Relatively safe chemicals may become toxic if the dose is high enough, and even potent, highly toxic chemicals may be used safely if exposure is kept low enough. 

The actual health risk of a given chemical is a function of both the toxicity of the chemical and dose (or exposure) someone has to that chemical:

Risk = Dose x Toxicity

No matter how toxic a chemical may be, there is little risk involved unless it enters the body. It is important to be aware of the routes of exposure (see above) for each chemical, and to protect those routes using appropriate control measures.

Evaluating Toxicity Data

Most estimates of human toxicity are based on animal studies, which may or may not relate to human toxicity. In most animal studies, the effect measure is usually death. This measure of toxicity is often expressed as LD50 (lethal dose 50), which is the dose required to kill 50% of the test population. The LD 50 is measure in milligrams of material per kilogram of body weight of the test animal. The LC50 (lethal concentration 50) may be used to determine the concentration in air that is lethal to half the population. 

Many factors influence the susceptibility of an individual to the effects of toxic substances:

• Nutritional habits,
• Physical condition,
• Medical conditions,
• Drinking and/or smoking,
• Pregnancy, etc.

Due to individual variation and uncertainties in estimating human health hazards, it is difficult to determine a dose of chemical that is completely risk-free. Regular exposure to some substances can lead to the development of an allergic rash, breathing difficulty, or other reactions. The phenomenon is referred to as sensitization. Over time, these effects may occur with exposure to smaller and smaller amounts of the hazardous material but may disappear soon after the exposure stops.

After becoming informed about the toxicity and health hazards of a chemical, you need to know the safe level for exposure. There are both regulatory levels and recommended guidance:

• OSHA's Permissible Exposure Limit (PEL) (regulatory level)
• ACGIH's Threshold Limit Value (TLV)* (recommended)
• NIOSH's Recommended Exposure Limit (REL) (recommended)

*TLVs may be found on Safety Data Sheets or in the ACGIH book (updated annually).

Most exposure limits are expressed as a time-weighted average (TWA). The TWA is the total amount of a substance that a person may be exposed to averaged over an 8-hour work day. Some substances may have a ceiling, which indicates the maximum dose of a substance a person can be exposed to at any time. If a person exceeds the ceiling, but doesn't exceed the TWA, there has still be an unacceptable exposure. Similar to a ceiling, there may also be a short-term exposure level (STEL). The STEL indicates the maximum dose of a substance a person can be exposed to in a 15-minute period.

Both regulatory and recommended exposure limits are based on exposures to the "average individual." Your personal reaction to a chemical may differ from that of the average person based on your medical history, circumstances of the exposure, your susceptibility, etc. Therefore, it is important that you familiarize yourself with the signs and symptoms of exposure to the chemicals you use. This way, you will be able to recognize if the symptoms start occurring.

So, why are some of the regulatory limits different than recommended limits?

OSHA regulations have not changed much since their effective date. As new research and data become available, the ACGIH have published new recommendations for exposure limits. While OSHA's PELs are the regulatory limits, UVM strives to keep exposures to any hazardous material as low as possible.

UVM labs are required to have copies of Safety Data Sheets (SDSs) available for fast review, either in the lab safety notebook or linked to your chemical inventory on Scishield. Lab personnel should be trained to review SDS information before using a chemical in the lab. SDSs may be audited during a lab inspection.

The Hazard Communication Standard (29 CFR 1910.1200(g), revised in 2012, requires that chemical manufacturers, distributors, or importers must provide SDSs for each hazardous chemical to users to communicate hazard information. Information contained in the SDS is required to be presented in a 16-section format with the following types of information:

• Properties of the chemical or solution
• Physical, health, and environmental hazard information
• Protective measures
• Safety precautions for handling, storing, and transporting the chemical

All hazard symbols and phrases should be consistent with the Globally Harmonized System of Classification and Labeling of Chemicals (GHS). Always check for a revision date to ensure the information is as updated as possible.

For more information about what each section on a Safety Data Sheet means, see this OSHA Brief (PDF).