The peaceful streets of Oslo are growing even calmer as the city drives noisy machines off municipal building sites. For locals and builders, the drop in decibels is a welcome side-effect of a goal to keep city-managed construction projects free from toxic emissions. The mandate, which is the first of its kind in the world, came into effect on 1 January .Construction is one of the biggest sources of urban air pollution, but even forward-thinking cities such as Oslo have struggled to clean it up.