By Brandon E. Boor, Ph.D.A, B

A. Lyles School of Civil Engineering, Purdue University, 550 Stadium Mall Drive, West Lafayette, IN 47907
B. Ray W. Herrick Laboratories, Center for High Performance Buildings, Purdue University, 177 South Russell Street, West Lafayette, IN 47907

“You know what I am? I’m a dust magnet!” enthusiastically proclaimed Pig-Pen, a beloved character in the popular comic strip, Peanuts, by Charles M. Schulz. It is said that Schulz drew inspiration for the character from observing his own son’s propensity for being surrounded by dirt1. Indeed, if you take a magnifying glass to inspect the air around your body, you will find millions of tiny airborne particles floating about. While Schulz may not have been an aerosol scientist, his portrayal of Pig-Pen as being perpetually enveloped by a cloud of particles is quite accurate.

Our activity patterns and movements indoors are responsible for creating personal clouds of dust. As we walk across the carpet or move in bed, we continually stir-up indoor dust – complex deposits of settled particles and their associated biological, chemical, and elemental material. This process, referred to as resuspension, is a major source of coarse-mode (> 1 µm) particles in our homes and offices, particularly particles of biological origin, such as bacteria, fungi, and allergens.

The quality of indoor air can be significantly influenced by human-driven resuspension of settled dust. An adult walking across the floor can resuspend 10 to 100 million particles per minute2, while tossing and turning on a mattress can stir-up similar levels of microbial-laden dust3. The episodic release of dust to air is an inherently transient process, changing minute by minute due to human occupancy and movement patterns.

While we have insights into how adults kick-up particles indoors, we know comparatively less about how small infants resuspend dust. As toddlers crawl, play, learn to walk, and move during sleep we would expect them to generate their own personal dust clouds (Figure 1). Given the close proximity of their breathing zones to flooring and bedding surfaces, they are likely to be exposed to elevated concentrations of resuspended particles in their near-surface microenvironments. Resuspension may contribute meaningfully to early-childhood inhalation exposures to the microbial and allergenic content of indoor dust. Such exposures can play a significant role in both the development of, and protection against, asthma, hay fever, and allergies.

Working with microbiologists and aerosol scientists in Finland, my research group recently published two of the first studies investigating dust clouds around crawling infants4, 5. To characterize the infant Pig-Pen effect, we developed a simplified mechanical crawling infant that bravely explored the particle plumes released from carpets. We used a laser-induced fluorescence (LIF)-based aerosol instrument to monitor biological particles in the infant dust clouds in real-time, along with off-line DNA-based analysis to probe specific bacterial and fungal taxa. We discovered that the crawling motion of an infant delivers a substantial number of resuspended particles to their breathing zones (Figure 2), with concentrations ranging from 50 to 600 µg/m3(number range of 0.5 to 2 cm-3). Infants create very concentrated clouds of dust around themselves, with breathing zone concentrations roughly ten times greater than those in the bulk air of the room.

Crawling-induced resuspension is a highly transient process, as illustrated in the size distribution time-series plot of Figure 2. An abrupt burst of resuspended fluorescent biological aerosol particles in the infant breathing zone can be observed at the onset of crawling. The particle concentration then fluctuated with respect to time during the remainder of the resuspension period and decreased rapidly following the cessation of crawling. The accelerated decay in particle concentrations post-crawling suggests that an infant will receive much of their exposure to self-induced resuspended dust during periods where they are actively engaged in locomotion.

We found most of the resuspended particles to be larger than 1 µm in diameter. The size distributions exhibited a prominent mode between 3 and 5 ?m, while a partial second mode was observed for particles greater than 10 µm. Given the results of our DNA-based analysis, we believe the 3 to 5 ?m mode is likely comprised of bacterial cell agglomerates. For the particular carpet samples studied, the most abundant bacterial genera in resuspended dust included: Oxalobacteraceae gen., Acinetobacter, Paracoccus, Corynebacterium, Micrococcus, Propionibacterium, Staphylococcus, Streptococcus, Clostridium, and Sediminibacterium.

The detailed size-resolved concentration data enabled us to predict how many resuspended biological particles deposit in the infant respiratory system per unit time crawling. In one minute of crawling across a carpet, on the order of 1,000 to 10,000 resuspended particles will deposit in the infant respiratory system. Infants will receive much of their inhaled dose in their lower airways – the tracheobronchial and pulmonary regions of their lungs.

Much like Pig-Pen, babies are surrounded by clouds of dust they resuspend from indoor surfaces. As house dust is heavily enriched with an amazing diversity of microorganisms, much of the resuspended particles are of biological origin. Understanding sources of indoor particles, and their contributions to human exposure, is an important step in working towards indoor environments that promote human health and well-being.

  1. Pugh, T. (2017). Exploring a Beloved Peanuts Character: New Exhibition at the Charles M. Schulz Museum. Press release by the Charles M. Schulz Museum and Research Center: June 28, 2017.
  2. Qian, J., Peccia, J., and Ferro, A.R. (2014). Walking-Induced Particle Resuspension in Indoor Environments. Atmospheric Environment. 89:464-481.
  3. Boor, B.E., Spilak, M.P., Corsi, R.L., and Novoselac, A. (2015). Characterizing Particle Resuspension from Mattresses: Chamber Study. Indoor Air. 25(4):441-456.
  4. Wu, T., Täubel, M., Holopainen, R., Viitanen, A.-K., Vainiotalo, S., Tuomi, T., Keskinen, J., Hyvärinen, A., Hämeri, K., Saari, S.E., and Boor, B.E. (2018). Infant and Adult Inhalation Exposure to Resuspended Biological Particulate Matter. Environmental Science and Technology. 52:237-247.
  5. Hyytiäinen, H., Jayaprakash, B., Kirjavainen, P., Saari, S.E., Holopainen, R., Keskinen, J., Hämeri, K., Hyvärinen, A., Boor, B.E., and Täubel, M. (2018). Crawling-Induced Floor Dust Resuspension Affects the Microbiota of the Infant Breathing Zone. Microbiome. 6:25.