Parking Garage Gas Detection: CO and NO2 Sensors, Code Requirements, and Best Practices

Gas detection is a critical component of any parking garage ventilation system, yet it is also one of the most commonly misunderstood. Beyond protecting occupants, properly specified sensors play a direct role in meeting mechanical code requirements and controlling ventilation systems efficiently reducing both safety risks and long-term energy costs.

IMC Code Requirements for CO and NO₂ Detection

Under the International Mechanical Code (IMC) Section 404.1, enclosed parking garages must either operate ventilation systems continuously or use automatic controls based on gas detection. In IMC 2021 and later editions, this automatic control method requires carbon monoxide (CO) detectors used in conjunction with nitrogen dioxide (NO₂) detectors. These devices must be UL 2075 listed and installed in accordance with their listing and manufacturer instructions.

ASHRAE Guidance Supports Code Compliance

Engineers frequently reference ASHRAE guidance alongside the IMC because ASHRAE standards—such as Standard 62.1—provide the engineering framework for ventilation rates, air quality control, and energy efficiency. While the IMC defines minimum code requirements, ASHRAE guidance helps engineers implement demand-controlled ventilation strategies that respond effectively to CO and NO₂ levels under real-world parking garage conditions.

For parking garages specifically, ASHRAE has identified CO and NO₂ as two of the most abundant contaminants and emphasized that controlling their levels is central to safe ventilation design, particularly for systems that vary airflow based on actual gas concentrations. In most applications, both CO and NO₂ sensors provide an effective coverage radius of approximately 50 feet. This distance is driven more by gas behavior and dispersion patterns than by the specific sensor brand or model, making proper placement just as important as product selection.

Different Gases, Different Hazards

One of the most common challenges in parking garage design is distinguishing between similarly named gases. Carbon monoxide (CO) and nitrogen dioxide (NO₂) are the primary gases of concern in garage environments, but they are often confused with other compounds associated with indoor air quality applications.

Carbon dioxide (CO₂), for example, is widely used as an indicator of occupancy and ventilation effectiveness in office buildings, schools, and other occupied spaces. In those environments, CO₂ levels rise as people occupy a space, making it useful for demand-controlled ventilation tied to comfort and air freshness rather than acute toxicity. Similarly, nitric oxide (NO) is typically referenced in specialized industrial, laboratory, or combustion analysis applications, not general parking garage safety.

Parking garages present a different risk profile. CO is produced primarily by gasoline-powered vehicles, while NO₂ is a byproduct of diesel engines. These gases can accumulate rapidly in enclosed or partially enclosed garages and pose direct health hazards if not properly controlled. Understanding these distinctions ensures gas detection strategies align with actual garage conditions and current mechanical code intent.

Commissioning and Code Considerations

Commissioning requirements are becoming more common and can significantly impact project cost and timeline. While the IMC does not define specific gas concentration set points, it does require ventilation systems to respond appropriately when unsafe CO and/or NO₂ levels are detected. Local jurisdictions often add amendments that establish trigger thresholds, which may require calibration gas, test kits, and additional labor during startup. Identifying commissioning and testing requirements early in the design phase helps avoid unexpected costs later in the project.

Sensor Placement and Installation Best Practices

Mounting height was once a point of debate, but current best practices have simplified installation. Today, both CO and NO₂ sensors are commonly mounted at the same height, typically 4 to 6 feet above the floor at breathing level. As NO₂ cools, it settles to levels similar to CO, making dual-gas sensors a practical and cost-effective solution. Using a single device to detect both gases can reduce installation time, wiring complexity, and overall system cost while still meeting IMC requirements.

Choosing the Right Sensor

Larger or more complex parking garages often benefit from controller-based gas detection systems, particularly when multiple zones, staged ventilation, or advanced monitoring and alarming are required. These systems offer flexibility, scalability, and centralized control for demanding applications. Smaller or less complex garages may be well served by standalone sensors that meet UL listing and code requirements while integrating directly with ventilation equipment. Selecting the right solution for the size and complexity of the space supports code compliance, occupant safety, and long-term system performance without adding unnecessary complexity.