User:Carolinefortune/Vapor intrusion

Causes and Process
=== Vapor intrusion is mainly caused by nearby chemical usage and improper cleanup of brownfield sites. This allows these chemicals to seep into soil or groundwater, and eventually end up in overhead building air. VOCs are most able to travel through porous soils because of the space it allows for the chemicals, and then diffuse in the vadose zone of soil, eventually to areas underneath manmade structures and buildings. Once under building foundation, the processes of advection and diffusion are responsible for the compounds traveling through cracks of the foundation until the gaseous compounds mix with the components of the indoor air. The ability of the compound to enter a building through advection, can be related to the pressure gradient of the soil versus the building, which is caused by the 'stack effect' of the foundation. It is important to note the effect "preferential pathways" have on the process of vapor intrusion. Preferential Pathways, such as elevator shafts, pipes, and storm drains, act as an easy path for VOCs to travel through via groundwater or soil gas, and enter a building through man-made entrances like vents and faucets. The usage of preferential pathways for Vapor Intrusion can be mitigated with well-kept sewer and pipe systems that do not allow contamination through cracks, leaks, and holes. ===

Concerns and Mitigation
=== Vapor intrusion can be a cause for concern when chemicals seep into areas of human residence or work. When these chemicals mix with the indoor air, they can lead to acute or chronic health problems, like headaches, mental status changes, and increased risk of certain cancers. Vapor intrusion can also pose a threat when an infiltrated building contains flammable materials, as many VA compounds can act as a catalyst to cause an explosion. With this said, it is important mitigation strategies are put in place where there is high risk of VOC intrusion (within 100 feet of a contaminant). In 2012 the EPA released a "Citizens Guide to Vapor Intrusion," where different mitigation methods are explained. Two methods in particular are described, sub-slab depressurization, which entails using a fan to vent chemical vapors outdoors, and the over-pressurization of buildings, in which building pressure is increased to decrease the pressure gradient between under the building and the inside of the building. For larger buildings, an architectural mitigation strategy that is noted to decrease the likelihood of chemicals traveling into indoors is large, ventilated structures being installed under buildings. These structures, such as parking garages, will allow the gaseous chemicals to disperse in outside air rather than traveling directly indoors. ===