CO and CO2 Monitoring for Workplace Safety: MOM Requirements and Equipment

Carbon monoxide (CO) and carbon dioxide (CO2) are two of the most common and dangerous invisible gas hazards in Singapore workplaces. CO is acutely toxic even at low concentrations, while elevated CO2 causes cognitive impairment and can be fatal at high levels, yet neither has any smell or colour to warn occupants. Under the Workplace Safety and Health Act (WSHA) and its subsidiary regulations, Singapore employers are legally required to assess and control exposure to these gases. Understanding what the law requires, what the exposure limits are, and how to select appropriate monitoring instruments is essential for facilities managers, safety officers, and engineers.

The Hazards of CO and CO2

Carbon monoxide is produced by incomplete combustion of any carbon-containing fuel (petrol, diesel, LPG, natural gas, charcoal, wood), binding to haemoglobin with roughly 240 times the affinity of oxygen and preventing oxygen transport. Symptoms progress from headache and dizziness at low concentrations to unconsciousness and death at high concentrations, often with no warning since CO impairs cognition before loss of consciousness. Common Singapore sources include underground and multi-storey carparks, enclosed loading bays with diesel forklifts, commercial kitchens with gas cooking, generator rooms, boiler rooms, and any enclosed space where combustion occurs with limited ventilation. Carbon dioxide is a naturally occurring combustion and metabolic byproduct, present outdoors at roughly 420 ppm, accumulating indoors from occupant respiration, fermentation, dry ice and combustion; unlike CO it's not directly toxic to tissue but acts as an asphyxiant and respiratory stimulant at elevated levels. Effects scale with concentration: below 1,000 ppm is acceptable indoor air quality (minor complaints possible above 800 ppm); 1,000–2,000 ppm brings drowsiness and reduced cognitive performance; 2,000–5,000 ppm brings headaches, reduced alertness and increased heart rate; above 5,000 ppm is the MOM TWA limit with significant health effects on prolonged exposure; and above 40,000 ppm (4%) is immediately dangerous to life and health. CO2 also serves as an indoor air quality proxy, since elevated CO2 indicates insufficient fresh air dilution — referenced by BCA Green Mark and NEA IAQ guidelines.

Singapore Regulatory Framework

The Ministry of Manpower (MOM) sets permissible exposure limits (PELs) for workplace chemical agents under the WSHA (Chemical Agents) Regulations. The current Singapore PELs are:

These PELs align closely with the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit values (TLVs) that Singapore has adopted as the basis for its regulations. TWA is the time-weighted average concentration that workers may be repeatedly exposed to over an 8-hour workday, 5 days per week, without adverse health effects. STEL is the maximum concentration permitted for any 15-minute period, provided the TWA is not exceeded.

For carparks specifically, Singapore's Building Control Act (administered by BCA) and the LTA carpark design standards require CO monitoring and mechanical ventilation control in enclosed and semi-enclosed carparks. The typical required CO alarm setpoint for automatic ventilation activation in Singapore carparks is 25–50 ppm, with higher-level alarms at 100 ppm.

Sensor Types for CO and CO2 Monitoring

Electrochemical CO sensors, the dominant CO detection technology, oxidise CO at a working electrode to generate a current proportional to concentration — highly specific, sensitive below 1 ppm, cost-effective, used in both portable and fixed controllers, though sensor life is typically 2–3 years, performance degrades with high humidity, hydrogen gas and some solvent vapours, and annual or more frequent calibration is recommended. NDIR CO sensors measure infrared absorption at CO's specific wavelength (~4.6 micrometres), more stable over time with lower cross-sensitivity, better suited to harsh industrial environments and preferred where long sensor life and minimum maintenance matter. NDIR CO2 sensors, used in virtually all commercial CO2 monitors, absorb infrared strongly at ~4.26 micrometres, offering 5–15 year operating life depending on quality, used in IAQ monitors, HVAC BMS sensors, incubators, greenhouses and industrial process monitors — many include automatic baseline correction (ABC) that corrects long-term drift by assuming the sensor occasionally sees fresh outdoor air (~400 ppm), which can cause low readings in continuously occupied spaces without fresh-air purges, worth noting during commissioning and calibration. Photoacoustic CO2 sensors modulate the IR source and detect the resulting pressure wave, offering very high accuracy and low cross-sensitivity for high-precision laboratory and reference instruments.

Fixed versus Portable Monitors

Fixed CO controllers in carparks, boiler rooms and generator rooms give continuous 24/7 monitoring with alarms and relay outputs activating ventilation fans, closing dampers or triggering BMS alerts. A standard Singapore carpark installation spaces CO sensors per the ventilation design (typically 1 per 300–500 m² of floor area), with a central controller panel, relay outputs to fan control at alarm threshold, and audible/visual high-level alarms. Fixed CO2 sensors in offices and meeting rooms connect to HVAC BMS systems to modulate fresh air supply in demand-controlled ventilation, reducing energy use while maintaining air quality, a key feature of BCA Green Mark and NEA IAQ-certified buildings. Portable instruments handle initial site surveys, commissioning checks, confined space pre-entry testing and personal monitoring — multi-gas portable detectors for confined space entry typically include CO alongside O2, H2S and combustible gas sensors, while standalone CO2 handhelds serve HVAC engineers for IAQ surveys and ventilation commissioning.

Calibration, Installation and Maintenance

CO and CO2 instruments need calibration at manufacturer-specified intervals dictated by regulatory requirements: electrochemical CO sensors every 3–6 months using certified calibration gas, NDIR CO2 sensors every 6–12 months (with zero point checkable against outdoor air as a field check between formal calibrations), all calibrations using NIST- or NPL-traceable certified gas mixtures, and records retained for MOM inspection. Unitest Instruments provides gas instrument calibration under SAC-SINGLAS accreditation (LA-2023-0845-C), accepted by MOM inspectors and ISO 45001 auditors, with 3–5 working day turnaround and on-site calibration for installed systems. For CO monitoring in carparks and enclosed spaces: mount sensors at breathing height (1.5–1.8m) where workers may be present, or at exhaust pipe height (0.3–0.5m) near ventilation dead-spots for vehicle exhausts; avoid direct ventilation airstreams that dilute concentrations before detection; test alarm relay outputs monthly; and replace electrochemical sensors at end of rated life (typically 2–3 years) regardless of apparent performance. For CO2 monitoring in offices: mount at occupant breathing height (1.1–1.5m) away from supply air diffusers and exterior walls, and verify the ABC calibration assumption is valid for the space, since spaces with no outdoor air purge periods may need manual calibration against a reference standard.

Contact Unitest Instruments for CO and CO2 instrument supply, calibration scheduling, and advice on MOM-compliant monitoring programmes.