The exposure to particulate matter has been consistently associated to respiratory and cardiovascular illness. Particulate matter is ubiquitous and exposure to it is both short term and long term depending on the microenvironment. In developing countries most of the exposure to particulate matter takes place in indoor settings, due to amount of time spent indoors. However, the development in transportation, in the last few decades, has resulted in increased mobility of people and a large proportion of the working population spends a significant time commuting. So in-transit microenvironment is emerging as an important microenvironment to calculate total human exposure to particulate matter.