Tunnel gas monitors

The importance of gas monitoring in tunnels

While 21st century fuel-efficient vehicles emit significantly less toxic gas and particulate matter than previously, it is still vitally important to measure emissions in road tunnels using purpose-designed tunnel gas monitors and have an efficient ventilation control system in place to protect drivers and ensure that that tunnel atmosphere remains safe.
The most common noxious gases that require monitoring road tunnels are:

• Nitrogen dioxide (NO2)
• Nitrogen monoxide (NO)
• Carbon monoxide (CO)

Carbon monoxide (CO)

Until recently, most tunnel ventilation systems were designed to operate based on levels of CO, NO and visibility within the tunnel bore. Currently all vehicles that run on petrol are fitted with a catalytic converter which significantly reduces CO emissions, while diesel engines inherently emit low levels of CO. As a result, the level of CO in a road tunnel rarely reaches the threshold required to activate the tunnel’s ventilation system.

Nitrogen dioxide (NO2) & nitrogen monoxide (NO)

NO and NO2 are pollutants created by internal combustion processes — (gasoline and diesel fuels). Unfortunately, the technology used in diesel powered cars to treat exhaust gases before they are emitted into the atmosphere (the DPF) tends to elevate the levels of NO2 in road tunnels.
While the level of NO typically found in road tunnels is not generally considered to be dangerous, historically it was always measured as an indicator of NO2 because NO oxidises into NO2 in the presence of oxygen. However, research indicates that inferring NO2 levels from NO measurements may not always be accurate.
Nitrogen dioxide is a toxic gas. Breathing air with a high concentration of NO2 can irritate airways in the human respiratory system. Short term exposure in tunnels can aggravate respiratory diseases (including asthma) which can result in coughing and breathing difficulties.

International standard & regulation for NO2 levels in tunnels

It is widely acknowledged that NO2 is dangerous to human health, but there is no consensus on the exact effects on the general population and no clear guidance on exposure limits. Many countries are yet to establish a NO2 standard in road tunnels and where limits are specified, they are generally between 0.4ppm and 1ppm.
The US National Institute for Occupational Safety proposes a 15-minute short-term NO2 limit of 1ppm. The World Road Association (PIARC) has proposed a permissible average tunnel concentration of 1ppm NO2 along the length of the tunnel.
To ensure that drivers and tunnel workers are protected and that all potentially harmful gas levels are accurately monitored, tunnel operators around the world specify Acoem’s various tunnel gas monitors for precise, reliable and cost-effective data that seamlessly communicates a tunnel’s SCADA system to enable ventilation systems to be run efficiently and safely.