It is a common statistic when talking about the quality of indoor air, that we spend approximately 90% of our time indoors. The origin of that statistic was a study by the US Environmental Protection Agency back in 19891. The importance of this is the nature of the contained environment of the home; in the move towards greater environmental sustainability, homes have become more energy efficient and more tightly sealed. While this provides a better environmental impact, it also means that the air in the home does not become renewed through passage of fresh air.