Safety in road tunnels and ventilation: an approach to transient pollutant concentration during accidental conditions.

The purpose of a ventilation system is to dilute harmful compounds deriving from exhaust gas from vehicles, avoiding injuries to the health of tunnel users and maintenance personnel. Three types of operational modes are considered for road tunnel ventilation: normal, emergency and temporary ventilation. In case of low profile events, the transient behaviour of the pollutant concentration can be faced by the analytical model here presented. It allows obtaining useful indications to prevent or mitigate the occurrence of noxious health effects. Dealing with high profile events, the basic scientific problem to be solved is the selection of the longitudinal ventilation rate necessary to prevent the combustion products from moving upstream of the fire. To a practical purpose and with a conservative approach, the problem is solved determining the critical velocity connected to the worst conditions, referred to a pool fire extended to the whole tunnel.