example, raw materials must be extracted from the earth, transported and then manufactured into building materials and transported again to be sold and finally transported to the build site. A lot of fossil fuels may be used during each of these stages. Earth sheltering often requires heavier construction materials to resist the weight of the earth against the walls and/or roof. Reinforced concrete in particular needs to be used in much larger quantities per building. The manufacture of concrete is a major source of greenhouse gases. The materials involved tend to be non-biodegradable substances. Because the materials must keep water out, they are often made of plastics. The excavation of a site is also drastically time- and labor-consuming. Overall, the construction is comparable to conventional construction, because the building requires minimal finishing and significantly less maintenance. Moisture and indoor air quality Problems of water seepage, internal condensation, bad acoustics, and poor indoor air quality can occur if an earth shelter has not been properly designed and ventilated. Very high humidity levels can allow mold or mildew growth, associated with a musty smell and potentially with health problems. The below-ground orientation of many earth-sheltered homes can allow accumulation of radon gas (which is known to increase the risk of lung cancer) or other undesirable materials (e.g. off gassing from construction materials). The threat of water seepage occurs around areas where the waterproofing layers have been penetrated. Earth usually settles gradually. Vents and ducts emerging from the roof can cause specific problems due to the possibility of movement. Precast concrete slabs can have a deflection of 1/2 inch or more when the earth/soil is layered on top of them. If the vents or ducts are held rigidly in place during this deflection, the result is usually the failure of the waterproofing layer. To avoid this difficulty, vents can be placed on other sides of the building (besides the roof), or separate segments of pipes can be installed. A narrower pipe in the roof that fits snugly into a larger segment of the building can also be used. The threat of water seepage, condensation, and poor indoor air quality can all be overcome with proper waterproofing and ventilation. Limited natural light Despite large windows (usually facing south in the Northern Hemisphere), many earth-sheltered homes have dark areas in the areas opposite the windows. All natural light coming from one side of the home