Toxic Substances

Many chemicals used in laboratories may be toxic. Toxicity depends on the quantity, frequency and duration of exposure, or the dose. Toxic effects can be local (at the site of exposure) or systemic (at another site in the body), acute or chronic. When dealing with toxic chemicals, it is important to consider the route of exposure and the target organ to ensure that proper controls are adhered to.

When planning and conducting experiments, it is critical to recognize that the combination of toxic effects of two or more substances may be significantly greater than that of one substance alone. Toxic reaction products can be much more dangerous than the starting reagents. Many cases of toxic reaction products occur when chemicals are mixed unintentionally. This can occur due to improper labelling, mishandling of chemicals and spills.

Lab workers must be able to recognize the potential hazards present to prevent exposure to toxic chemicals, and in the event of accidental exposure, know the procedures for emergency, spill response and first aid.

Dose-Response Relationship

" All substances are poisons; there is none which is not a poison. The right dose differentiates a poison..." (Paracelsus).

The single most important factor that determines whether a substance is harmful is the relationship between the amount/concentration and the toxic effect it produces. This is referred to as the dose-response relationship. Many chemicals are toxic with one small exposure, while others require repeated doses (chronic exposure). It is important to remember that this relationship is unique to each chemical.

The toxicity of a chemical is best identified by its lethal dose 50 (LD₅₀). LD₅₀ is defined as the amount of a chemical that when given to a specified laboratory animal (e.g. rat), it will kill 50% of the animals. The LD₅₀ is usually expressed in mg/kg or g/kg of body weight. Thus, chemicals with low LD₅₀'s are highly toxic, requiring a low dose to cause fatal effects. For chemicals where the hazard is inhalation, the LC₅₀ (lethal concentration 50), which is the concentration of chemical in air, is more often used. LC₅₀ is usually expressed in ppm, mg/L or mg/m³. This information is found on the MSDS.

Acute and Chronic Effects

The duration and frequency of exposure are factors in whether a chemical will produce a toxic effect. While effects can occur after a single dose (acute), or with intermittent (repeated), or long-term (chronic) exposures, the specific chemical and route of entry determine the effect. Some chemicals are acutely toxicic, while others require repeated exposures or have long-latency effects (effect appears long after the exposure).

Generally, the longer the duration and/or frequency of exposure, the greater the toxic effect. For certain chemicals, the body is able to eliminate the toxins and/or repair tissues to some degree, thus the time between exposures will also affect the total dose required for a response.

Routes of Exposure

Ingestion - refers to exposure by entrance through the mouth and swallowing. The substance enters the gastrointestinal tract and is absorbed into the system through the blood stream. Ingestion is a method of exposure that is highly preventable through use of proper personal protection equipment, personal hygiene practices and adherence to rules regarding eating, drinking and use of cosmetics in the lab.

Inhalation - gases, vapours, aerosols, particulates, fibers and dusts can enter the body and cause either local or systemic effects by absorption through the mucous membrances in the mouth, throat and lungs.

For gases, the most important factor in rate of absorption is solubility. Chemicals that are highly soluble easily penetrate the lung tissue and blood, which allows them to be transported to other organs. For solids, particle size determines where a particle can penetrate and how far into the respiratory tract it can travel. Solubility of the particle (fat or water-soluble) also determines whether it will be cleared from the body, or remain for long periods of time causing chronic or long-latency disease.

A major factor in inhalation is the vapour pressure of the chemical - the higher the vapour pressure, the greater the potential of it being released into the air. It is also important to remember that vapour pressure increases with temperature, therefore by heating a substance, the likelihood that hazardous vapours will be produced increases. Procedures to control the production or release of airborne toxins are important to prevent both acute and chronic exposures. Ventilation through the use of fume hoods is the key control measure in preventing inhalation exposure. It is also necessary to reduce the surface area of solvents exposed, to cover containers, and to follow procedures to avoid spills.

Absorption - can occur through intact or broken skin, or via a body orifice (e.g. eye, ear). Either the skin itself may be injured, or the chemical may be absorbed into the body and transported via the bloodstream. Use of PPE such as lab coats, gloves and goggles are the best method of protection against absorption.

Injection - needles, razor blades, broken glass or other sharp objects that pierce the skin can allow toxic chemicals or biohazardous agents to enter the body. Proper handling and disposal of sharps is necessary to eliminate this method of exposure.

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