The Swiss Alpine region is particularly vulnerable to acidification. High NOx and ozone loads are caused by long-range transboundary air pollution originating from the industrial zones of northern Italy and from urban zones of Switzerland itself. Trans-Alpine traffic (tourism and trucks) is also a major pollution source. The Alpine cantons are confronted with forest damages as a result of sulphur depositions and ozone concentrations. Air pollution does not primarily threaten vital economic interests, but could jeopardise the equilibrium of ecosystems and the security of Trans-Alpine traffic routes. Alpine forests are essential for the protection against soil erosion, falling rocks and avalanches. Furthermore, they provide resources for the local economy (timber) and recreation areas (tourism). During the 1970s, damages also became visible in urban areas in the form of air pollution from stationary sources and seasonal smog. Later on, high ozone concentrations shaped the public debate. Clean air discussions primarily focused on human health, but increasingly shifted to eco-system damages.