The public is being assured the air in two state highway tunnels in Wellington is safe to breathe.
The Transport Agency says monitoring shows air quality in the Mt Victoria Tunnel and Terrace Tunnel has improved substantially since 2003.
It says air quality in both tunnels is well inside World Health Organisation guidelines.
The agency's state highway manager for Wellington, Rob Whight, says though they acknowledge the air can be smelly, it is safe to breathe.
Mr Whight credits an improvement largely to newer cars and trucks with better emissions.
The Transport Agency says air quality is likely to continue to improve due to this, and planned tunnel upgrades, in the next decade.
Green Party co-leader Russel Norman says he is surprised by the results of the monitoring, but it is good to hear the tunnel air is not as bad as people think.