Methods to assess PM2.5 exposure from indoor sources in epidemiological studies: a review
Sandoval Diez, Nekane
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Of all air pollutants, particles with a diameter of less than or equal to 2.5 μm (PM2.5) have the greatest scientific evidence of adverse health effects, including respiratory, cardiovascular, nervous system effects, cancer, and mortality. Epidemiological studies are an invaluable and very important tool to establish the toxicity and effects of PM2.5 on human health. Most epidemiological studies have measured or modelled ambient concentrations of PM2.5 to estimate human exposure. However, humans spend most of their time indoors, where almost all exposure to air pollutants takes place. Indoor PM2.5 concentration is a mixture of both ambient and non-ambient particles. The ambient component refers to all the particles generated outdoors that infiltrate into the indoor environment through, for example, ventilation. Nonambient particles refer to all those particles that are produced directly indoors or by human personal activities. Many human activities can be sources of indoor PM2.5, although the most important are smoking, cooking, and heating. Outdoor and indoor PM2.5 sources are different, so the particle's composition, toxicity, and human health effects may also be distinct. Therefore, it is important to treat exposure to outdoor- and indoor-generated PM2.5 as two separate exposures to investigate their effects in epidemiological studies. To separate indoor- and outdoor-generated PM2.5 exposure, researchers have used tracer compounds whose production occurs almost exclusively outdoors and infiltrate into indoor environments, a method called source partitioning. However, the separation between indoor- and outdoor-generated PM2.5 implies many logistical challenges for epidemiological research since it often entails a great economic cost for the researchers to carry out measurements in each of the residences or indoor environments. It can also be inconvenient for the participants. This is why most of the studies that measure indoor PM2.5 concentrations do so in small study populations or for short periods of time. Having small study populations limits the ability of epidemiological studies to draw valid results. In this literature review, we aimed to evaluate the approaches and techniques used to assess indoor-generated PM2.5 exposure in epidemiological studies of respiratory health effects, pointing out their strengths and limitations. We identified a total of 29 epidemiological studies that carried out measurements of indoor PM2.5 concentrations, of which only 5 applied methods to separate indoor- and outdoor-generated particles. All studies that applied source partitioning methods used sulphate or iron as tracer compounds, had small study populations, and studied short-term exposures. The results of studies applying source partitioning methods highlight the importance of investigating the specific relationship between indoor-generated PM2.5 and respiratory health outcomes, as well as distinguishing it from outdoor sources.