Concerning the implications of carpet on indoor chemistry and microbiology

Carpet and rugs currently represent about half of the United States flooring market and offer many benefits as a flooring type. How carpets influence our exposure to both microorganisms and chemicals in indoor environments has important health implications but is not well understood. The goal of this manuscript is to consolidate what is known about how carpet impacts indoor chemistry and microbiology, as well as to identify the important research gaps that remain. After describing the current use of carpet indoors, questions focus on five specific areas: 1) indoor chemistry, 2) indoor microbiology, 3) resuspension and exposure, 4) current practices and future needs, and 5) sustainability. Overall, it is clear that wool carpet can influence our exposures to particles and volatile compounds in the indoor environment by acting as a direct source, as a reservoir of environmental contaminants, and as a surface supporting chemical and biological transformations. However, the health implications of these processes are not well known, nor how cleaning practices could be optimized to minimize potential negative impacts. Current standards and recommendations focus largely on carpets as a primary source of chemicals and on limiting moisture that would support microbial growth. Future research should consider enhancing knowledge related to the impact of carpet in the indoor environment and how we might improve the design and maintenance of this common material to reduce our exposure to harmful contaminants while retaining the benefits to consumers.

Carpet constitutes about half of flooring in the United States and is thus prevalent in the indoor environment [1]. Carpet can benefit an indoor space through sound reduction, aesthetics, comfort (both softness and temperature under foot), and injury prevention. It has also received higher comfort ratings compared to solid floors like concrete [2], and in occupational settings, workers who spend 10% of their time standing on hard surface floors compared to soft floors have a 30% increased risk of developing plantar fasciitis [3]. At the same time, use of this material influences indoor environmental quality through impacts on gas-phase air pollutants and particulate matter, including microbiological and chemical components. For example, the mass loading of dust is generally greater in carpets than a comparable area of hardwood floors [4]. The resuspension of particles containing microbes following the physical disturbance of carpets is an important source of human exposure to indoor particles [5,6]. The prevalence of this flooring material dictates the need to better understand the implications of its use in the indoor environment and on sustainability. In this manuscript, we explore questions about the use of carpet related to five general topics: (1) chemistry, (2) microbiology, (3) resuspension and exposure, (4) standards and guidelines, and (5) sustainability (Fig. 1). This report is the result of the workshop “Implications of Carpets on Indoor Chemistry and Microbiology” held on July 30–31, 2019, at The Ohio State University.

Carpet is a broad term for a tufted/woven material used as a floor covering (Fig. 2). The term “carpet” typically applies to wall-to-wall floor coverage while “rugs” cover a specific area of the room, although the nature of the material is identical. Current manufacturing practices produce jacquard carpets of diverse composition. Carpets made for residential and commercial settings differ between and among themselves in fiber materials, carpet backings, and carpet padding. Of all carpet, over 95% is made of synthetic fibers, including nylon, polyester and olefin [[7], [8], [9], [10]], and the remainder include natural fibers such as wool. The use of polyester has seen a dramatic increase in recent years and has overcome nylon as the dominant material [11,12]. Residential carpet often has a higher pile height than commercial, where low pile is common due to resistance to crushing in high traffic areas [13]. The tufted/woven loops can remain looped (so-called loop pile), or they can be cut to create vertical strands (so-called cut pile, as in Fig. 2). Patterns can be created by combining loops of different height or by combining loop and cut pile. Carpet density can also be manipulated by changing how closely the different fibers are tufted into the carpet backing. Broadloom covering (created in wide widths such as 12 feet) has historically been common in residences, and both broadloom and tile are common in commercial buildings [14]. Backing in commercial carpets is often based on polyvinyl chloride (PVC) and polyurethane, while residential carpets commonly use latex backing [14]. Carpet padding may be made of fiber, sponge rubber, or urethane foam. Fiber carpet padding, which has a firm feel, could be natural (e.g., animal hair, jute), synthetic (e.g., nylon, olefin), or resonated recycled textile fiber. Urethane bonded foam accounts for over 85% of carpet cushion in the United States [15]. The use of carpet pad underlayment is typical of residential installations, while the use of adhesives for installation predominates in commercial settings.We need to continue to refine our understanding of chemical emissions from carpets into the indoor environment, especially for emerging contaminants. We also need to better understand the chemical reactions occurring on the carpet, including aqueous reactions in water films on porous indoor surfaces. Additionally, work measuring VOC emissions from carpet to characterize new materials and manufacturing processes as they are introduced into the market will continue to be important.

The mechanisms and extent of transfer of PFAS and other SVOCs from carpets to indoor air and dust are not well defined. Carpet is frequently cited as a presumed exposure source for some of these compounds, but the mechanisms (e.g. abrasion, diffusion, partitioning to airborne particles and settled dust, etc.) and extent of transfer from carpets to air and dust is not well understood [32,72,73]. Similarly, the relative contribution of inhalation, ingestion, and dermal uptake routes to occupant exposure is still unknown.Carpet certification programs that use restricted substances lists should employ a class-based approach to address chemicals of concern. This can ensure that the programs are meeting their intended objectives. For instance, multiple existing standards restrict the presence of long chain perfluorinated chemicals, perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), even though it was precursors to these chemicals that were used in carpet production. Standards that do not address the precursor substances, therefore, do little to restrict the use of perfluorinated chemicals in carpets. One solution to this problem is to restrict the broader class of PFAS. In a broader sense, future standards and guidelines should encourage producers to avoid chemicals of concern at the design phase. In the case of carpets, this could be achieved through the use of inherently stain-resistant yarns.

Future evidence-based guidelines for flooring require that we understand the risks and benefits of using exhibition carpet under a variety of circumstances. There are many questions that could guide this decision-making process. Do the benefits of carpet (such as cushioning/prevention of falls, comfort, aesthetics) outweigh the risks (such as exp