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 (CO)

Carbon monoxide is produced by the incomplete combustion of any carbon-containing fuel — petrol, diesel, LPG, natural gas, charcoal, and wood. It binds to haemoglobin in the blood with approximately 240 times the affinity of oxygen, preventing oxygen transport. Symptoms progress from headache and dizziness (at low concentrations) to unconsciousness and death at high concentrations or with prolonged exposure, often with no warning to the victim because CO causes cognitive impairment before loss of consciousness.

Common Singapore workplace sources include:

  • Underground carparks and multi-storey carparks with petrol/diesel vehicles
  • Enclosed loading bays with diesel forklifts or delivery vehicles
  • Commercial kitchens with gas cooking equipment
  • Generator rooms and back-up power installations
  • Boiler rooms and furnace areas
  • Any enclosed space where combustion occurs with limited ventilation

Carbon Dioxide (CO2)

CO2 is a naturally occurring combustion product and metabolic byproduct, present in ambient outdoor air at approximately 420 ppm (0.042%). In enclosed or poorly ventilated spaces, it accumulates from occupant respiration, fermentation processes, dry ice operations, and combustion. Unlike CO, CO2 is not directly toxic to tissue but acts as an asphyxiant and respiratory stimulant at elevated concentrations.

Effects by concentration:

  • Below 1,000 ppm — acceptable indoor air quality; minor complaints possible above 800 ppm
  • 1,000–2,000 ppm — drowsiness, reduced cognitive performance, poor air quality complaints
  • 2,000–5,000 ppm — headaches, reduced alertness, increased heart rate; typical MOM TWA limit zone
  • Above 5,000 ppm — MOM TWA exposure limit; significant health effects with prolonged exposure
  • Above 40,000 ppm (4%) — immediately dangerous to life and health (IDLH)

CO2 monitoring is also used as an indoor air quality (IAQ) proxy — elevated CO2 indicates insufficient fresh air dilution of occupant-generated pollutants. Singapore's BCA Green Mark scheme and NEA IAQ guidelines reference CO2 as a key IAQ indicator.

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:

GasTWA (8-hour)STEL (15-minute)Ceiling
Carbon monoxide (CO)25 ppm100 ppm—
Carbon dioxide (CO2)5,000 ppm30,000 ppm—

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.

CO Monitoring Equipment and Sensor Types

Electrochemical CO Sensors

The dominant technology for CO detection. An electrochemical cell oxidises CO at the working electrode, generating a current proportional to CO concentration. Electrochemical CO sensors are highly specific to CO, sensitive to concentrations below 1 ppm, and cost-effective. They are used in both portable personal monitors and fixed wall-mounted controllers.

Limitations: sensor life is typically 2–3 years; performance degrades with high humidity, hydrogen gas, and some solvent vapours; sensors must be stored and operated within the temperature range specified by the manufacturer. Annual or more frequent calibration is recommended.

Non-Dispersive Infrared (NDIR) CO Sensors

NDIR sensors measure CO concentration by the absorption of infrared light at CO's specific absorption wavelength (approximately 4.6 micrometres). They are more stable over time than electrochemical sensors, have lower sensitivity to interfering gases, and are better suited to harsh industrial environments. NDIR sensors are used in high-end fixed CO controllers and analytical instruments. They are the preferred technology for applications requiring long sensor life and minimum maintenance.

CO2 Monitoring Equipment and Sensor Types

NDIR CO2 Sensors

Virtually all commercial CO2 monitors use NDIR technology — CO2 absorbs infrared light strongly at approximately 4.26 micrometres. NDIR CO2 sensors are accurate, stable, and have operating lives of 5–15 years depending on quality. They are used in indoor air quality monitors, HVAC BMS sensors, incubators, greenhouses, and industrial process monitors.

Modern NDIR CO2 sensors include automatic baseline correction (ABC) algorithms that correct for long-term drift by assuming that the sensor occasionally encounters fresh outdoor air (approximately 400 ppm). In continuously occupied spaces without regular fresh-air purges, ABC may cause the sensor to read low — something to be aware of in commissioning and calibration.

Photoacoustic CO2 Sensors

Photoacoustic sensors modulate the IR source and detect the pressure wave (sound) generated by CO2 absorbing the light. They offer very high accuracy and low cross-sensitivity. They are used in high-precision laboratory and reference instruments.

Fixed versus Portable CO/CO2 Monitors

Fixed Monitoring Systems

Fixed CO controllers in carparks, boiler rooms, and generator rooms provide continuous 24/7 monitoring with audible/visual alarms and relay outputs to activate ventilation fans, close dampers, or initiate building management system alerts. A standard Singapore carpark installation includes:

  • CO sensors spaced according to the ventilation system design (typically 1 sensor per 300–500 m2 of floor area, or per the carpark engineer's design)
  • A central controller panel with LCD display showing CO concentration for each zone
  • Relay outputs to fan control panels for ventilation activation at alarm threshold
  • Audible horn and visual beacon for high-level alarm

Fixed CO2 sensors in offices, meeting rooms, and classrooms connect to HVAC BMS systems and modulate fresh air supply in demand-controlled ventilation (DCV) systems — reducing energy consumption while maintaining air quality. This is a key feature of BCA Green Mark and NEA IAQ-certified buildings.

Portable CO/CO2 Monitors

Portable instruments are used for 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. Standalone CO2 handheld meters are used by HVAC engineers for IAQ surveys and ventilation commissioning. View Unitest Instruments' range of gas measurement instruments.

Calibration Requirements for CO and CO2 Instruments

CO and CO2 instruments require calibration at intervals specified by the manufacturer and dictated by the application's regulatory requirements. For MOM-regulated workplace monitoring:

  • Electrochemical CO sensors should be calibrated every 3–6 months using certified calibration gas
  • NDIR CO2 sensors should be calibrated every 6–12 months; the zero point can be checked against outdoor air as a field check between formal calibrations
  • All calibrations should use certified calibration gas mixtures with NIST- or NPL-traceable concentrations
  • Calibration records must be retained and produced on MOM inspection

Unitest Instruments provides gas instrument calibration services under SAC-SINGLAS accreditation (LA-2023-0845-C). Our calibration certificates are accepted by MOM inspectors and ISO 45001 auditors. Calibration turnaround is 3–5 working days, with on-site calibration available for installed systems.

Installation and Maintenance Best Practices

For CO monitoring in carparks and enclosed spaces:

  • Mount sensors at breathing height (1.5–1.8 m) in areas where workers may be present; for vehicle exhausts, mount at exhaust pipe height (0.3–0.5 m) near ventilation dead-spots
  • Avoid mounting sensors directly in ventilation airstreams that would dilute concentrations before detection
  • Test alarm relay outputs monthly and document the result
  • Replace electrochemical sensors at the end of their rated service life (typically 2–3 years) regardless of apparent performance

For CO2 monitoring in offices and meeting rooms:

  • Mount at occupant breathing height (1.1–1.5 m) away from supply air diffusers and exterior walls
  • Verify ABC calibration assumption is valid for the space — 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.