Modern, fuel-efficient vehicles may emit significantly less toxic gas and particulate matter than earlier models, but it is still vitally important to monitor emissions in road tunnels and have an efficient ventilation control system in place to protect drivers and ensure the tunnel atmosphere remains safe.

The most common noxious gases in road tunnels are:

  • Nitrogen Dioxide (NO2)
  • Nitrogen Monoxide (NO)
  • Carbon Monoxide (CO)

Historically, 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 catalytic converters which significantly reduce 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.

NO and NO2 are pollutants from the internal combustion process. While the level of NO typically found in road tunnels is not considered to be dangerous, it was always measured as an indicator of NO2 as NO oxidises into NO2 in the presence of oxygen.

Testing has shown that inferring NOlevels from NO measurements can be inaccurate.

Nitrogen Dioxide is a toxic gas. Breathing air with a high concentration of NOcan irritate airways in the human respiratory system. Short term exposure can aggravate respiratory diseases (including asthma) which can result in coughing and difficulty breathing.

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 NOin road tunnels.

It is widely acknowledged that NOis dangerous to human health, but there is no consensus on the exact effects on the general population and no clear guidance on exposure limits.

As a result, many countries have not set a NOstandard 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 NOlimit of 1ppm. PIARC proposed to permit an average tunnel concentration of 1ppm NOalong 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 VICONOX as the most effective solution to measure CO, NO, NO2, NOx, visibility and temperature. VICONOX provides precise, reliable and cost-effective data to a tunnel’s SCADA system facilitating the ventilation system to be run efficiently and safely.

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