Atmospheric testing is the single most common compliance failure point CCL Health & Safety encounters in Ontario confined space programs, and it is consistently among the leading contributors to confined space fatalities investigated by the Ministry of Labour, Immigration, Training and Skills Development. The regulation is not complicated; the practical execution is where programs slip. This guide is written for Ontario employers and health and safety officers who already understand the basics and need a clear answer to the question: what does Ontario require for atmospheric testing in a confined space, and how do you actually do it correctly. It is written from over 20 years of CCL practitioner experience building programs in industrial and agricultural workplaces under both Ontario Regulation 632/05 and the federal framework, and it cites the regulation directly rather than paraphrasing it.
What Ontario Regulation 632/05 Requires for Atmospheric Testing
Ontario Regulation 632/05 is the confined space regulation made under the Occupational Health and Safety Act. The atmospheric testing requirements sit within Sections 11 through 18 of the regulation. Read together, those sections establish the floor for what every Ontario employer must do before and during a confined space entry.
The structural requirements are straightforward. Atmospheric testing must be carried out before any worker enters the confined space, and again during the entry whenever conditions could change while work is in progress. The testing must be performed using equipment that is suitable for the hazards in question, that has been properly calibrated, and by a person trained in its use. Test results must be recorded as part of the entry permit documentation required under Section 10.
The regulation organises atmospheric hazards into three categories that every Ontario employer needs to be able to test for:
- Oxygen content outside the acceptable range. Below 19.5 per cent oxygen creates a deficiency that impairs cognition and motor function. Above 23 per cent creates an enriched atmosphere that dramatically lowers the ignition threshold of clothing, hair, and otherwise non-flammable materials.
- Flammable or combustible accumulation. Concentrations of vapours or gases that could ignite or explode. The regulation requires this to be tested as a percentage of the lower explosive limit (LEL).
- Atmospheric contaminants. Exposure to airborne chemicals (gases, vapours, dusts) at concentrations exceeding applicable occupational exposure limits.
Exposure limits for the contaminant category are set out in Ontario Regulation 833 (Control of Exposure to Biological or Chemical Agents), not in O. Reg. 632/05 itself. Reg. 833 maintains the table of Time Weighted Average (TWA) and Short-Term Exposure Limit (STEL) values that apply to Ontario workplaces, and it is updated periodically. Any confined space program needs to identify which contaminants from the Reg. 833 table could be present in the specific space and ensure the monitoring instrument can measure them.
Read the full regulation at ontario.ca/laws/regulation/050632. For a section-by-section walkthrough of the broader regulation, see Ontario Regulation 632/05 Explained.
Acceptable Atmospheric Levels for Confined Space Entry
The regulation states the categories. Practitioner practice in Ontario, supported by CSA Z1006:23 and the manufacturer specifications of every modern multi-gas instrument, has converged on the following working thresholds for general industrial confined space entry.
Oxygen
Acceptable oxygen content is between 19.5 and 23 per cent by volume. Normal ambient air sits at 20.9 per cent. Oxygen below 19.5 per cent is a deficiency and is the most common cause of confined space fatalities in Canada. Oxygen above 23 per cent is enrichment and is often missed because it does not feel different to the worker. Enrichment is treated as a stop condition because of the increased ignition risk it creates.
Lower Explosive Limit (LEL)
LEL readings express flammable gas concentration as a percentage of the concentration at which ignition becomes possible. CCL's working rule for cold work entry is below 10 per cent LEL, with most practitioner programs in Ontario setting their entry threshold lower still where ventilation cannot guarantee a stable reading. Hot work (welding, grinding, cutting) cannot proceed in a confined space until the atmosphere is essentially clean of flammable gases, with most programs requiring 0 per cent LEL plus continuous monitoring.
Carbon Monoxide
Ontario's 8-hour Occupational Exposure Limit for carbon monoxide under Reg. 833 is 25 ppm. Many Ontario programs set the confined space entry alarm at the same threshold to align field practice with the regulatory exposure limit. CO is colourless, odourless, and produced by combustion engines, gas-fired heaters, and many process operations. Engine exhaust drifting into a confined space (a vehicle idling near an entry hatch, a generator running upwind) is a common contamination path.
Hydrogen Sulphide
Reg. 833 sets the H2S 8-hour OEL at 10 ppm. H2S is heavier than air and pools at the bottom of the space, which makes top-only sampling especially dangerous. The "rotten egg" odour is detectable at sub-ppm levels but olfactory fatigue causes the smell to disappear at higher concentrations, so smell is not a reliable indicator. H2S is common in agricultural manure pits, sewage and wastewater work, oil and gas operations, and certain food and beverage process spaces.
Other contaminants
For specific spaces with known chemical hazards (ammonia in food and beverage refrigeration, chlorine at municipal water plants, methane in agricultural digesters, solvent vapours in coatings or printing operations), the standard 4-gas instrument is not sufficient. The hazard assessment must identify which Reg. 833 contaminants are plausibly present and the monitoring instrument must include sensors for them.
Pre-Entry vs Continuous Monitoring: When Each Is Required
Pre-entry testing is mandatory before any worker enters. The intent is to verify that the atmosphere is safe at the moment of entry, that ventilation has done its job, and that no unexpected contaminant is present. A pre-entry test is not optional regardless of how routine the entry is or how recently the space was last entered.
Continuous atmospheric monitoring is required during the entry whenever conditions inside the space could change while work is ongoing. In CCL's Ontario practice the following five scenarios always trigger continuous monitoring:
- The work itself can change the atmosphere (welding, cutting, coating, abrasive blasting, solvent cleaning, any process producing fumes or vapours).
- Adjacent process activity could introduce contamination (an adjoining tank in service, a connected vent line, a process unit upstream).
- Ventilation is being relied on to maintain acceptable atmospheric conditions during the entry.
- The space contains residual material (sludge, scale, residue) that could off-gas as it is disturbed.
- The duration of the entry exceeds the time it takes for stable conditions to be assumed (in practice, almost any work entry beyond a quick inspection).
In every case, samples must be drawn from the top, middle, and bottom of the space. Gases stratify by density: heavier gases settle to the floor and lighter gases rise to the ceiling. A single reading from head height misses both. The hazard assessment for each space should specify the sampling strategy, including which sample points are accessed by drop sampling tubes and which by physical access.
Instrument Calibration and Bump Testing
These two procedures are confused more often than any other atmospheric testing topic. They are different procedures with different purposes and different frequencies.
A bump test is a brief functional verification. Test gas of a known concentration is briefly applied to each sensor. The expected response is that the sensors register the expected readings within tolerance and the alarms activate. A bump test typically takes under a minute. It does not adjust the instrument; it verifies the instrument is responding correctly. CSA Z1006:23 and manufacturer guidance both call for bump testing before each day of use.
A calibration is a full procedure that exposes each sensor to a certified gas standard and adjusts the instrument's readings to match. Calibration restores measurement accuracy. The frequency is set by manufacturer specifications and the regulatory baseline in Ontario is to follow those specifications. For most modern multi-gas instruments, that means full calibration at least annually, with semi-annual or quarterly schedules common in heavier usage environments.
Common calibration and bump test failures observed in CCL's practice:
- Instruments used past their manufacturer-specified calibration interval. Sensor drift is gradual and not visible to the operator.
- Bump testing skipped because the instrument "looks fine." A failed sensor will read zero in a contaminated atmosphere with no warning.
- Calibration gas cylinders past their fill date, producing inaccurate calibrations that then propagate as inaccurate field readings.
- Calibration certificates not retained, leaving no documentary evidence that the instrument was within calibration at the time of a particular entry.
Documentation requirements for calibration and bump testing align with the broader recordkeeping requirements of O. Reg. 632/05. Calibration records and bump test logs must be retained for the instrument and made available on request. If a serious incident is investigated, these records are among the first documents the inspector will ask for.
Multi-Gas Monitor Selection for Ontario Workplaces
The standard multi-gas instrument for general Ontario industrial confined space work is a 4-gas monitor measuring oxygen (O2), lower explosive limit (LEL), carbon monoxide (CO), and hydrogen sulphide (H2S). For most general manufacturing, food and beverage processing, municipal utility work, and farm operations, this configuration covers the typical hazard set.
A 4-gas instrument is not enough when the hazard assessment identifies any of the following:
- Volatile organic compounds (VOCs). Solvents, coatings, fuel residues, paint vapours. A photoionization detector (PID) sensor is required to measure total VOC concentration.
- Specific toxic contaminants. Ammonia (NH3) in refrigeration spaces, chlorine (Cl2) at water and wastewater facilities, sulphur dioxide (SO2) in process spaces, nitrogen dioxide (NO2) in spaces with combustion engines or welding.
- Methane in agricultural or sewage settings. While LEL sensors detect methane, they do not distinguish among flammable gases. In some agricultural confined spaces a dedicated methane sensor adds confidence.
Sensor types matter for selection. Catalytic bead sensors measure LEL by combusting the sample in the sensor, which means they require oxygen to function and can be poisoned by silicones. Infrared (IR) LEL sensors function in oxygen-deficient atmospheres and resist poisoning, making them the better choice in inerted spaces. Electrochemical sensors measure specific gases (CO, H2S, O2, ammonia, chlorine, and others) and are the standard for toxic gas detection. Photoionization sensors measure VOCs by ionizing the sample with UV light.
Internal pump versus diffusion sampling is the second selection question. A diffusion instrument samples the atmosphere at the instrument's physical location only. An instrument with an internal pump and a sampling tube can draw samples from a remote location, which is required for pre-entry testing of a space the worker has not yet entered (the instrument samples through the entry point before the entrant goes in). For most confined space programs in Ontario, an instrument with both modes available is the practical choice.
Recording and Documenting Test Results
Atmospheric test results must be recorded as part of the entry permit documentation required under O. Reg. 632/05. The minimum information that must be captured for each entry includes:
- The instrument used (make, model, and serial number)
- The most recent calibration date and the most recent bump test date for that instrument
- The test results from each sample point (top, middle, bottom), including oxygen, LEL, CO, H2S, and any additional contaminants identified in the hazard assessment
- The time of pre-entry testing
- For continuous monitoring entries: a record of any alarm activations, the readings at the time of alarm, and the response taken
- The name of the person who carried out the testing
These results become part of the entry permit record. The minimum retention recommended in CCL practice is the longer of one year, or until two of the most recent records of that type exist for the instrument and the space. Many Ontario employers retain entry permits for the full period of the program to support due diligence and incident investigation.
For multi-employer workplaces, atmospheric monitoring documentation responsibility usually rests with the employer doing the entry. The constructor or owner of the space (the lead employer under O. Reg. 632/05 Section 4) typically requires copies of the testing records and calibration certificates for the integrated workplace file before authorising the entry.
Atmospheric data is one of ten sections every defensible Ontario permit captures. For the full ten-section walkthrough see What Goes Into an Ontario Confined Space Entry Permit. For the fall protection requirements that apply when the entry involves a vertical drop, see Fall Protection for Confined Space Entry in Ontario.
Common Gas Monitoring Compliance Failures
Five failures account for most of the gas monitoring compliance gaps CCL identifies in Ontario program audits. Each is preventable, and each has appeared in cases where atmospheric conditions contributed to a fatality or serious incident.
- Skipping pre-entry testing because the space "looked safe." The most common shortcut. A space that appears unchanged from a prior safe entry can have new contamination from process drift, stored material decomposition, or adjacent work. Pre-entry testing is required regardless of how routine the entry is or how recently the space was last tested. Prevent this by making the permit system require recorded pre-entry test results before the permit is signed off.
- Using uncalibrated or expired instruments. The instrument may read zero across all sensors and give the entry team confidence that does not match the actual conditions in the space. Sensor drift is gradual; a failing sensor does not announce itself. Prevent this by gating instrument issuance on a current calibration certificate and a bump test that day.
- Single-point sampling instead of stratified sampling. A reading at the entry hatch does not reflect conditions at the floor where H2S accumulates, or at the ceiling where methane collects. Sample top, middle, and bottom of the space using a drop tube where physical access is not possible. Document each sample point on the permit.
- Not recording results, or recording only summary data. A permit that says "atmosphere acceptable" with no readings is not compliant evidence. Record the actual readings for each sample point, with the time of the test and the instrument identifier. This is what the inspector or the auditor will look for.
- Continuous monitoring that is not actually continuous. A worker carrying a multi-gas monitor on a clip is not the same as continuous monitoring if no one is responding to the alarms or watching the readings change. The attendant must be in communication with the entrant and watching the trend. Prevent this by training the attendant on the specific instrument and documenting alarm response procedures in the entry plan.
Most of these failures are documentation and procedure failures rather than knowledge failures. A program that captures the right data at the right point in the workflow tends to surface and eliminate these issues before they become an incident. The Canadian Centre for Occupational Health and Safety publishes practical guidance on confined space atmospheric testing at CCOHS confined space atmospheric testing.
Federal Workplaces and Part XI of COHSR
Federally regulated workplaces in Canada (banks, telecom, federal Crown corporations, ports, inter-provincial transport, licensed grain elevators, and some agricultural operations connected to inter-provincial trade) fall under Part XI of the Canada Occupational Health and Safety Regulations rather than O. Reg. 632/05. The atmospheric testing requirements in Part XI parallel the Ontario framework but use slightly different terminology and section numbers, and the federal framework distinguishes between a confined space and a hazardous confined space (see the resource on Part XI of COHSR for that distinction).
The federal regulation is published at laws-lois.justice.gc.ca COHSR. If your workplace is unsure which framework applies, see Provincial vs Federal Confined Space Regulations for the jurisdictional decision tree.
How CCL Health & Safety Helps
CCL builds confined space programs to O. Reg. 632/05 and Part XI of COHSR for Ontario industrial and agricultural workplaces. We carry out site-specific hazard assessments, develop atmospheric monitoring procedures matched to the space and the work, train workers and supervisors on instrument use, and produce permit and recordkeeping templates that align with regulator expectations. For ongoing program management, CCL maintains the program, updates it on the review cycle, and supports incident investigation when atmospheric conditions are in question. The Reg. 833 exposure limits used in the contaminant category of the regulation are published at ontario.ca/laws/regulation/900833. For the broader regulatory context, see CSA Z1006:23 Explained for the Canadian standard for confined space management, or Confined Space Training and Programs for our service offerings, including program development engagements.