About $6 million is likely to be spent on overhauling ageing road tunnels in an attempt to keep within air quality guidelines. A review of ventilation and pollution monitoring equipment in all five government-run tunnels will begin this week. It will determine how much they lag behind levels set by the Environmental Protection Department. Electrical and Mechanical Services acting assistant director Patrick Lai Wai-hung said each tunnel could take up to six months to properly review. He added about $1.2 million could be spent on each one - the airport tunnel, Tseung Kwan O, Shing Mun, Aberdeen and the oldest, Lion Rock, which opened in 1967. Senior transport officer Honson Yuen Hong-shing said apart from ventilation systems, the study would look at installing sophisticated equipment to monitor particulates and emissions. He denied emission levels were a problem in the tunnels saying the department 'tried its best' to meet recommendations. 'We want to improve the environment even though it generally meets these standards,' Mr Yuen said. He added: 'Present practices are very different to when the tunnels were first opened. 'We have to study how much we need to upgrade and modernise the tunnels in this respect.' The guidelines control carbon monoxide, nitrogen dioxide and sulphur dioxide concentrations, along with smoke and particulate effects on visibility. Mr Yuen said existing equipment only monitored carbon monoxide levels and a more accurate picture was needed.