Particles in Indoor Air

Indoor air particles play a critical role in determining indoor air quality, which directly affects human health and well-being. These particles range in various sizes – from large visible dust particles to small stable gatherings of molecules. Indoor air quality refers to the condition of the air inside buildings, that includes homes, offices, schools, and other enclosed spaces. The study of modern indoor air science started in the 1970s and has since raised concern about indoor air pollution's potential health effects. Health-related studies have investigated the composition, sources, and behavior of indoor particulates and how it is connected to respiratory diseases, allergies, and other cardiovascular problems. Understanding the sources of indoor air particles is crucial to developing effective strategies to maintain a healthy indoor environment and improve the overall quality of life.

Provided below are the primary and secondary sources of particles that people are exposed to indoors.

Primary Source

Cooking

Particulate emissions from cooking come from two sources, 1) the heat source (e.g., stove, oven) and 2) the cooking process itself. The type of appliance and ventilation to the outside air influences the input of particles in the cooking area which may affect the overall indoor particle concentration. Heat from direct combustion, such as natural gas, propane, liquid petroleum gas (LPG), kerosene, and solid fuels (i.e., wood, coal, etc.) emit different ultrafine particle sizes. A properly tuned natural gas combustion produces particles with a geometric mean (GM) mobility diameter of 19.5 ± 1.4 nm, propane flame has a GM of 26.5 ± 1.3 nm, LPG creates 52 ± 1.2 nm particles, and solid fuels are reported to emit 1.23 ± 1.8 nm, 48 ± 1.9 nm, and 152 ± 1.9 nm particles for firewood, coal, and dung cake, respectively. Electrical cooking surfaces can also produce ultrafine particles from organic constituents that condense into the heating elements, which get volatilized and then nucleate to form less than 10 nm particles. Cooking food makes additional source of particles and often represents a primary short-term source of particle emissions. During frying in oil or melted solid fats above their smoke temperature, the water from the food vaporizes to form bubbles which then burst and eject tiny oil droplets into the air. Other cooking activities, such as boiling and broiling, also cause emissions of particles of different sizes.

Heating

For many houses, stoves are also used to heat the room. Thus, similar sizes of particles may be emitted into the indoor air space.

Cleaning

Paradoxically, cleaning aims to reduce the level of contamination in indoor spaces, but it could also pose as a source of particles in several ways. Brooming may cause resuspension of particles greater than 1mm. While, vacuuming may also increases particulate matter resuspension, which depend on the efficiency of dust bags (cloth bag vs. HEPA filter).

Lifestyle

Human daily activities, such as smoking, vaping, candle or incense burning, and use of spray products, can also introduce particles indoors. Electronic cigarettes (e-cigs), commonly called vaping, have increased in popularity recently as an alternative to tobacco cigarettes (t-cigs). The use of e-cigs produces similar high levels of fine and ultrafine particles (UFPs) as to t-cigs. Combustion of candles and incense can also produce high concentrations of particles. Candles with added scents elevate particle emissions by releasing volatile and semi-volatile organic compounds. Different types of wax and wick also affect the particle size distribution of emitted particles, usually in the range of 5.4 – 7.1 nm. Incense has lower combustion temperature which produces much larger particles with particle diameter size of around 136 nm. Other consumer products, such as air fresheners, spray cleaners, and other aerosolized liquids or solids, provide a source of particles indoors. In products with active solutions, particles may be created after the solvent has evaporated.

Infiltration of Ambient Aerosol

Outdoor particles transporting into indoor space is a major long-term source. The exchange can occur directly from ventilation or infiltration through gaps in the building envelope. Buildings can be ventilated in three ways: mechanical ventilation, natural ventilation, and infiltration. Mechanical ventilation includes the use of mechanical equipment such as fans. Opening the windows or doors offer natural ventilation. Infiltration of outdoor particles into the indoor environment occurs through unintentional openings in the building such as cracks and gaps.

Secondary Source

Indoor chemistry has a significant influence on the composition of indoor particles. Indoor air particles may arise from various chemical reactions and transformations that occur within confined spaces. These reactions can result in the emission of particles of varying sizes and compositions, leading to complex mixtures that may negatively impact human health. Some of the prominent secondary sources of indoor air include photochemical and chemical reactions. Photochemical reactions in indoor spaces are usually fueled by sunlight entering the building although light intensities are usually insufficient to drive the same types of photochemical reactions observed outdoors. There can be photochemical reactions where indoor pollutants, such as volatile organic compounds (VOCs) and nitrogen oxides (NOx), are exposed to light, they can absorb energy and become excited, which lead to the breaking of chemical bonds and formation of reactive intermediates or radicals that further reacts with molecules in the air forming new compounds or particles. Another pathway in creating particles in indoor environments is the presence of surfaces on which heterogeneous reactions can occur.

Reference: “Handbook of Indoor Air Quality, Volumes 1. https://doi.org/10.1007/978-981-10-5155-5 (and references therein)”