Health effects: terminology and exposure limits

Hazard is a substance’s potential to cause harm. Risk concerns the likelihood and severity of harm under particular exposure conditions. The chemical form, amount, route, frequency and duration of exposure all matter, along with individual susceptibility.

A hazardous substance does not cause the same risk in every situation. An element’s name or a hazard pictogram alone does not establish the risk from a particular drinking-water sample or workplace task.

Background: CCOHS — what makes chemicals poisonous.

Exposure can occur through inhalation, ingestion, or contact with the skin and eyes. Some chemicals can also be absorbed through the skin. A measured concentration in air or water is not the same quantity as the dose entering or absorbed by the body.

Acute exposure is short in duration; chronic exposure extends over a longer period, often through repeated contact. Short exposure can cause lasting harm, and effects can appear after a delay. Symptoms do not have to appear immediately for an exposure to matter.

A workplace air limit does not account for every possible route of exposure. Check skin-absorption and sensitisation notes and the substance-specific guidance.

A CAS Registry Number (CAS RN) is a numerical identifier assigned by CAS, Chemical Abstracts Service, to a substance in its registry. It helps distinguish substances that have several names.

The number does not encode a molecular structure, toxicity level, exposure limit or regulatory approval. CAS REGISTRY also covers substances such as polymers and materials of variable composition. Match the product identity, ingredients and chemical form to the relevant record; a trade name alone may describe a mixture.

Source: CAS Registry Number explanation.

A safety data sheet (SDS), historically called an MSDS, provides product-specific information on hazards, handling and exposure controls. GHS is the Globally Harmonized System of Classification and Labelling of Chemicals; legal implementation differs between jurisdictions.

Useful SDS sections include:

A hazard classification describes the type and evidence of a hazard; it does not quantify an individual’s risk at every concentration. Use the current SDS for the actual product and jurisdiction.

Reference: OSHA — SDS section requirements.

In Canadian drinking-water guidance, a maximum acceptable concentration (MAC) is a health-based guideline for a specified contaminant in drinking water. Its interpretation depends on the contaminant, sampling requirements and the applicable document.

Health Canada develops national guidelines with provincial and territorial partners. Legal requirements are established through the relevant jurisdiction’s legislation or regulatory framework; a guideline and an enforceable local standard are different categories.

Do not infer a generally permissible time to exceed a MAC. The appropriate response depends on the contaminant and local requirements. A result for one chemical also does not establish the overall suitability of water for drinking.

Sources: Health Canada — current summary tables and Ontario Drinking Water Quality Standards, O. Reg. 169/03.

Interim maximum acceptable concentration (IMAC) appears in older Canadian and Ontario drinking-water documents. It identified an interim value where the evidence or practical considerations did not support establishing a definitive MAC.

Treat the term as document- and date-specific. An IMAC found in an older table is not evidence that the same classification or numerical value applies today. Check the current contaminant guideline and applicable regulation.

Historical example: Ontario Regulation 459/00. For present requirements, consult the current sources linked under MAC.

Water: 1 mg/L = 1,000 µg/L. In dilute water with density close to 1 kg/L, 1 mg/L is approximately 1 ppm on a mass basis. For concentrated solutions, confirm density and the definition of ppm.

Air: ppm commonly expresses a gas or vapour volume fraction, while mg/m³ expresses mass per air volume. Converting between them requires the substance’s molar mass and specified temperature and pressure. A water concentration cannot be directly compared with an air exposure limit.

Check what was measured: total or dissolved material, the chemical species, and the reporting basis. Results reported “as N” and “as nitrate”, for example, use different mass bases.

“Not detected” is not a measurement of zero. Read the laboratory’s detection or reporting threshold, method and qualifiers. If that threshold is above the concentration of interest, a non-detect result may be insufficient for the intended assessment.

Examples of reporting guidance: US EPA — laboratory unit conversions and US EPA — interpretation of non-detect groundwater results.

A permissible exposure limit (PEL) is an enforceable occupational exposure limit under the applicable US Occupational Safety and Health Administration (OSHA) standard. Most OSHA PELs for air contaminants use an 8-hour time-weighted average; other provisions include ceilings, peaks and substance-specific short-term limits.

Check the exact standard, substance, jurisdiction and any skin or other notation. PELs are workplace limits, not drinking-water standards or general-population exposure guidelines.

Reference: OSHA — chemical hazards and exposure limits.

A time-weighted average (TWA) weights each measured concentration by the duration of that exposure over a specified reference period. For concentration C and time t:

TWA = Σ(C × t) / reference duration

An 8-hour TWA and a TWA based on another workday duration are different comparisons. Use the reference period and calculation rules specified by the source; missing measurements should not simply be assumed to represent zero exposure.

An acceptable long-period average does not override a STEL, ceiling or other short-term restriction. Average concentration alone can conceal brief high exposures.

A short-term exposure limit (STEL) controls exposure over a short averaging period, commonly 15 minutes. The exact period and any restrictions on repeated exposures depend on the substance and issuing authority.

A STEL may supplement a longer-period TWA or stand alone. Where both apply, both must be met. The timing and excursion guidance associated with one organisation’s system must not be treated as a universal rule for all workplace limits.

A ceiling value (C) is a concentration intended not to be exceeded during the working exposure. Read its prescribed measurement period and any specifically defined peak or excursion provisions in the applicable standard.

A ceiling and an 8-hour average answer different questions: a low daily average does not demonstrate that a ceiling was respected.

Threshold Limit Values (TLVs®) are ACGIH occupational-health recommendations, rather than regulatory standards in themselves. They can acquire legal relevance when adopted by a jurisdiction.

TLVs are not a sharp boundary between harmless and harmful exposure, and they do not protect every individual in every situation. Consult the current substance documentation, the applicable TWA, STEL or ceiling category, and any additional notations.

Source for TLV, STEL and ceiling terminology: ACGIH — introduction to chemical-substance TLVs.

Recommended exposure limits (RELs) are recommendations issued by the US National Institute for Occupational Safety and Health (NIOSH). Their averaging periods and notations must be read from the relevant substance entry; they should not automatically be treated as OSHA PELs.

Immediately dangerous to life or health (IDLH) values address high-risk airborne exposure conditions, including effects that could prevent escape. They support respiratory-protection planning and are not routine operating targets or assurances that exposure below them is harmless.

Sources: NIOSH Pocket Guide introduction and NIOSH IDLH guidance.

Primary references are linked with each topic. The earlier page also cited the ILPI MSDS HyperGlossary, retained here as supplementary terminology background. The official issuing authority is the source for current limits and legal requirements.

This page explains terms rather than providing a comprehensive limit table or a determination of whether a particular water supply or workplace is safe. Numerical limits, classifications and sampling rules can change. Compare the same substance or species, exposure route, units and reference period using the current applicable source.