Cold climates may present special challenges. Road salt contains chlorides that could migrate through the porous pavement into groundwater. Snow plow blades could catch block edges of concrete pavers or other block installations, damaging surfaces and creating potholes. Sand cannot be used for snow and ice control on porous surfaces because it will plug the pores and reduce permeability. Although there are design modifications to reduce the risks, infiltrating runoff may freeze below the pavement, causing frost heave. Another issue is spalling damage, which exclusively occurs on porous concrete pavement from salt application during the winter season. Thus porous paving is suggested for warmer climates. However, other materials have proven to be effective, even lowering winter maintenance costs by preserving salt in the pavement itself. This also reduces the amount of storm water runoff that is contaminated with salt chlorides. Pervious concrete and asphalt designed to reduce frost heave and spalling damage has been used successfully in Norway and New Hampshire. Furthermore, experience suggests that preventive measures with rapid drainage below porous surfaces be taken in order to increase the rate of snow melt above ground.