Ontario's Designated Substances regulation (O. Reg. 490/09) under the Occupational Health and Safety Act identifies 11 substances as "designated substances" — materials so hazardous that they require specific regulations, employer assessments, exposure controls, and medical surveillance programs. For construction workers, several of these substances are encountered regularly, sometimes without the worker even knowing it. Understanding what they are, where they appear on construction sites, and what the law requires is not optional knowledge — it is a survival skill.
All 11 are covered by one regulation, O. Reg. 490/09, but it does not apply to construction employers on a project. On construction projects, asbestos is governed by O. Reg. 278/05, exposure to the others is limited by Regulation 833, and section 30 of the OHSA requires the owner to list any designated substances present before the project begins.
The 11 Designated Substances
Here is the complete list of Ontario's designated substances, along with where construction workers are most likely to encounter each one.
Acrylonitrile — a chemical used in the manufacture of plastics, synthetic rubber, and adhesives. On construction sites, workers may encounter acrylonitrile in certain industrial coatings, adhesives, and sealants. It is a suspected human carcinogen and can be absorbed through the skin. Exposure typically occurs during renovation or demolition of industrial facilities where acrylonitrile-based products were used in manufacturing processes.
Arsenic — historically used in pressure-treated lumber (chromated copper arsenate, or CCA), pesticides, and certain paints. Construction workers encounter arsenic when cutting, sanding, or demolishing CCA-treated wood, which was widely used for decks, fences, playground structures, and foundation sheathing before its residential use was restricted in 2004. Inhaling sawdust from CCA-treated wood is the primary exposure route. Arsenic is a known human carcinogen.
Asbestos — the most commonly encountered designated substance on Ontario construction sites. Found in pipe insulation, floor tiles, ceiling tiles, drywall joint compound, spray-applied fireproofing, cement products, and electrical insulation in buildings constructed before 1990. Asbestos causes mesothelioma, asbestosis, and lung cancer. It is regulated under O. Reg. 278/05, which classifies asbestos work into Type 1, 2, and 3 operations based on risk level.
Benzene — a volatile organic compound found in petroleum products, solvents, and gasoline. Construction workers may be exposed to benzene when working with fuel-powered equipment in enclosed spaces, handling petroleum-based solvents, or performing work in refineries and petrochemical facilities. Benzene is a known cause of leukemia and other blood cancers. Even short-term exposure at high concentrations can be immediately dangerous.
Coke oven emissions — a complex mixture of gases and particulates produced during the coking of coal. While primarily an industrial hazard, construction workers performing maintenance, renovation, or demolition work at steel mills and coking facilities may be exposed. Coke oven emissions are a known cause of lung cancer, kidney cancer, and skin cancer.
Ethylene oxide — a gas used for sterilization in hospitals and medical device manufacturing facilities. Construction workers performing renovation or HVAC work in hospitals, laboratories, or sterilization facilities may encounter ethylene oxide. It is a known human carcinogen and reproductive toxin. Exposure can occur through inhalation and skin absorption.
Isocyanates — highly reactive chemicals used in spray-applied polyurethane foam insulation, two-part coatings, and certain adhesives. This is one of the most relevant designated substances for modern construction workers. Spray foam insulation applicators, painters using isocyanate-based coatings, and workers in the vicinity of spray foam application are all at risk. Isocyanates are the leading cause of occupational asthma in Ontario. Even a single high-exposure event can trigger permanent sensitization.
Lead — found in paint on buildings constructed before 1990 (and very likely in those built before 1960), in lead solder on copper plumbing, in lead flashing, and in certain industrial coatings. Construction workers are exposed during renovation and demolition when lead paint is sanded, scraped, cut with torches, or otherwise disturbed. Lead is a cumulative neurotoxin that causes damage to the brain, kidneys, and reproductive system. Children and pregnant workers are especially vulnerable.
Mercury — found in older thermostats, fluorescent lamps and tubes, certain switches, and some industrial instruments. Construction workers encounter mercury during demolition and renovation of older buildings when these devices are disturbed or broken. Spilled mercury vaporizes at room temperature and is readily inhaled. Mercury causes neurological damage, kidney damage, and tremors.
Silica (crystalline) — one of the most widespread and underestimated hazards on Ontario construction sites. Crystalline silica is found in concrete, brick, mortar, stone, sand, and grout. Any activity that cuts, grinds, drills, or crushes these materials generates respirable silica dust. Concrete cutting, masonry work, sandblasting, tuck-pointing, and demolition are all high-exposure activities. Silica causes silicosis (an irreversible scarring of the lungs), lung cancer, kidney disease, and COPD. Ontario's occupational exposure limits for respirable crystalline silica are 0.10 mg/m3 for quartz and 0.05 mg/m3 for cristobalite.
Vinyl chloride — a gas used in the manufacture of polyvinyl chloride (PVC) pipes and products. Construction workers may be exposed when cutting or heat-welding PVC pipe, particularly in poorly ventilated spaces. Vinyl chloride is a known human carcinogen associated with liver cancer (angiosarcoma). Exposure is most likely during industrial construction and maintenance at PVC manufacturing facilities.
Employer Assessment Obligations
On a construction project, section 30 of the OHSA puts this duty on the owner: before the project begins, the owner must determine whether any designated substances are present at the site and list them, and the list must reach the constructor and every contractor and subcontractor before they sign a contract. This is not a suggestion — it is a requirement that must be fulfilled before work begins.
Pre-construction assessment — before renovation, demolition, or maintenance work on an existing building, the owner must determine which designated substances are present, usually through a designated substance survey to identify the presence, location, and condition of any designated substances in the work area.
Qualified assessor — the assessment must be conducted by a competent person — typically an occupational hygienist or environmental consultant — who can identify designated substances, assess exposure risks, and recommend control measures.
Assessment report — the results must be documented in a written report that is available to the joint health and safety committee (JHSC) or health and safety representative, and to any worker who requests it.
Control program — if designated substances are present, the employer must control exposure with substitution, engineering controls, work practices and hygiene facilities, relying on personal protective equipment such as respirators only where those controls are not practical (Regulation 833 on most projects, O. Reg. 278/05 for asbestos). The formal control programs of O. Reg. 490/09, with air monitoring and medical surveillance, apply to other workplaces, not to construction employers on a project.
Exposure Limits
Each designated substance has an occupational exposure limit (OEL) set by Ontario regulation. These limits define the maximum concentration of the substance that a worker may be exposed to over an 8-hour time-weighted average (TWA) or a short-term exposure limit (STEL), typically measured over 15 minutes. Employers must ensure that exposures remain below these limits through engineering controls first, administrative controls second, and personal protective equipment as a last resort.
Air monitoring is how employers verify that control measures are effective and that worker exposures remain below the OEL, and sharing the results with workers and the JHSC lets everyone see whether the controls are working. On construction projects, testing is specifically required in some cases, such as checking carbon monoxide levels wherever an engine runs inside a building or excavation.
Medical Surveillance
In workplaces covered by O. Reg. 490/09, the control program for asbestos, benzene, coke oven emissions, isocyanates, lead, mercury or silica must include medical examinations paid for by the employer. That regulation does not apply to construction employers on a project, where Regulation 833 requires examinations at the employer's expense when a worker's health has been, or is likely to be, affected by an exposure. The specific requirements vary by substance but generally include:
Baseline examination — a medical examination before or at the start of exposure, including a detailed occupational and medical history, physical examination, and substance-specific tests (lung function testing for asbestos and silica, blood lead levels for lead, etc.).
Periodic examinations — follow-up examinations at regular intervals, typically annually or bi-annually, depending on the substance and level of exposure.
Exit examination — a medical examination when the worker leaves the job or when exposure ceases, to establish a baseline for future reference.
Record retention — medical records must be maintained for a minimum period specified in the regulation, typically 40 years for carcinogens. Workers have the right to access their own medical records.
Designated substances are not theoretical hazards. They are present on Ontario construction sites every day — in the concrete being cut, the insulation being sprayed, the old paint being scraped, and the buildings being torn down. Knowing what they are and what the law requires is the first step in making sure they do not end your career or your life.
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