1. Introduction
Indoor air pollution coupled with poor ventilation may cause serious problems, specifically for some women, children, and elderly who spend a great deal of time indoors. Most people spend approximately 80–95% of their time in indoor facilities breathing on average 10–14 m
3 of air per day [
1]. Worldwide, about 3.8 million premature deaths per year are attributed to indoor air pollution [
2]. Adverse health effects such as cardiovascular deaths, asthma, nervous system disorders, and stunted growth are among the many conditions associated with indoor air contamination [
2,
3,
4]. Indoor contaminants often get suspended in air, adhere to particulate matter, and then settle as dust covering furniture, floors, and other objects [
5].
Among the most prominent contaminants in indoor dust are heavy metals, which are non-degradable, toxic, carcinogenic, and have an acute or chronic impact on human health [
3,
4,
6,
7]. Household consumer products, electrical appliances, carpets, building materials, and other products serve as extensive sources of heavy metals in indoor dust [
5,
6,
8]. External sources of dust such as soil, road dust, and industrial and vehicular particulates often enter homes via the airborne route or are carried by inhabitants [
7]. Indoor floor dust acts as a major sink for airborne pollutants including a wide variety of organic and inorganic chemical contaminants in addition to bacterial, fungal, and viral contaminants [
4]. The quality of indoor dust often depends on the natural and anthropogenic sources of contamination, surrounding environment, economic development, and outdoor air quality. Hence, indoor floor dust can serve as a key indicator of indoor contamination and human health risks of a geographical region [
9].
Exposure to indoor dust can occur through inhalation, ingestion, and dermal routes [
10]. Children are the most vulnerable group affected by exposure to heavy metals in dust [
7] because of frequent hand-to-mouth movement and crawling on the floor. In humans, bioaccumulation of Cd causes kidney injuries, tumors and hepatic dysfunction, poor reproductive capacity, and hypertension [
11], while As causes cardiovascular disease, skin cancer, developmental anomalies, neurologic, neurobehavioral, and hematologic disorders [
12]. Accumulation of Cr affects the respiratory tract [
13] while Pb toxicity can affect nearly every organ and system in the body, slows growth, causes anemic seizures, and behavioral and learning difficulties in children [
14]. Consequently, USEPA revised the dust lead clearance levels (DLCL) from 40 μg/ft
2 and 250 μg/ft
2 to 10 μg/ft
2 and 100 μg/ft
2 for floors and windowsills, respectively [
14].
Various global studies have been conducted on the elemental and bacterial concentrations in indoor floor dust collected from households and other indoor environments in urban areas [
5,
7]. However, few are conducted in the United States and none were located in the Houston metropolitan region. The Houston metropolitan region suffers from poor air quality on account of industrial point sources such as: petroleum refineries, on-road emissions from motor vehicles and off-road emissions from ships, airplanes, freight trains, and construction activities [
15,
16]. Therefore, the objectives of the study are: (1) to identify and determine the selected elemental and bacterial concentrations in representative indoor dust samples from Harris County and surrounding areas, (2) to assess the effect of indoor habitat on the dust composition and characterize the samples using microscopy, and (3) to analyze the spatial distribution of contaminants and assess human health risks from indoor dust exposure.
4. Discussion
Concentrations of Cd, Cu, Ni, Pb, and Zn in our indoor dust samples exceeded the background soil limits (
Table 1). The higher concentration of metals in indoor dust over outdoor soil is attributed to the ability of organic rich indoor dust to accumulate metals over soil [
5]. The common sources for Cu in the house dust include building material and products such as pipes, electrical appliances, wires, and treated wood; Pb can be sourced to cheap metal jewelry, plastic toys, paints, and solder, Ni from stainless steel debris, Zn from rubber carpets, household appliances such as refrigerators, air conditioners and washing machines, and Cd from various plastics and batteries [
4,
5]. Other sources of metals in indoor dust include cooking, fuel combustion, smoking, candles, incense burning, shedding of fibers, furniture dust, and infiltration of outdoor particles [
5].
The higher concentration of Al in multi-family dwellings is likely from the ubiquitous use of Al wiring on older multi-family buildings. About 1.5 to 2 million single-family homes, mobile homes, and multiple-family dwelling built between 1965 and 1971 were wired with Al [
29]. Various other sources of Al in household dust include radios, toasters, electrical wiring, refrigerators, and personal care products [
30]. Elevated levels of Pb and Cd in older homes of over 30 years (
Table 1) could be due to deteriorating lead-based paint used in houses built before 1978 [
31]. Houses with uncarpeted floors showed a higher concentration of Zn, Pb, and Cd than both partial and 100% carpeted dwellings. This can be attributed to the elevated levels of Pb, Cd, and Zn in the vinyl and laminate floorings [
32]. Significant Cd concentrations in houses heated by gas is likely from the burning of fossil fuels [
33].
The concentrations of macro elements such as Na, Mg, K, Ca, and Mn (
Table 2) were higher than the heavy metals in indoor dust samples (
Table 1). The source of these elements includes various minerals such as quartz, albite, calcite, and dolomite, which are predominantly distributed in the outdoor dust of both urban and rural regions [
9]. The reasons for high concentration of enteric bacteria, Mn, and other elements in single-family homes over multi-family homes remains unclear to us. Perhaps, it can be attributed to carry over of outdoor dust by increased human activities and living style [
9]. However, dust samples from houses with pets showed higher total and enteric bacterial concentrations (
Table 2). This confirms that the bacterial and fungal richness and biodiversity is higher in the indoor dust of houses with pets [
34]. Cu, Fe, Zn, Pb, and Ni were significantly correlated (
Table 3) with each other indicating that these elements may have derived from the same or closely related sources. Cu, Fe, Zn are related to wear parts of alloys, used in machinery, lubricants, building materials, appliances, and wood products [
4].
The concentrations of Cd, Cr, Cu, and Pb were higher in the sample locations from urban centers (
Figure 2) compared to the sub-urban regions. Differing degrees of urbanization, vehicular traffic, road network, and economic development impacts the indoor and outdoor dust quality [
9]. The physical and chemical characteristics of our indoor dust samples were found to be different in the regions of rapid and slow development [
9]. In addition, indoor dust sampled from locations near major roadways was highly correlated to the outdoor particle pollution related to traffic [
2,
35]. We observed concentration of Zn in indoor dust to be higher in urban centers while the total and enteric bacteria concentrations were elevated in the suburban regions (
Figure 3). Perhaps this is because, apart from the outdoor air, bacterial concentrations in indoor dust depend on ventilation, occupant behavior and activities, pets, and building materials, etc. [
35]. Since dust samples were collected during the winter when homes are tightly sealed, we expected higher elemental concentrations. If additional samples were collected in the summer season, concentrations would be lower compared to the winter season as ventilation helps to remove or dilute indoor airborne contaminants.
The morphological and elemental composition of the dust samples, as determined by SEM and X-ray diffraction, revealed that it consisted of mainly carbon and oxygen in the particles identified as fiber, skin, sand, and salts (
Table 4;
Figure 4). The shape and size of the dust particles can also be used to identify the source of the dust. Vehicular area minerals have more irregular shapes compared to the industrial and residential area sources [
36]. Smaller dust particles possess higher air mobility, longer residence time in the atmosphere, and, hence, possess higher inhalation and ingestion risk affecting human health [
37,
38]. Several studies have suggested that the bioaccessibility and cytotoxicity of metals depend on their adsorption to various dust particles [
38].
The Enrichment Factor (EF) of As, Cr, Cu, Pb, and Zn was higher than one, indicating considerable enrichment in the household environment relative to their natural abundances (
Figure 5). The EF of Cu and Pb exceeded 20, indicating that the abundance of these elements was more concentrated in indoor dust than natural occurrence (
Figure 5). The Zn, Pb, and Cu showed the high EFs values in indoor dust at 166, 47, and 22, respectively (
Figure 5), suggesting anthropogenic sources were contributing to their release into the ambient environment. Vehicular traffic, road dust, electrical appliances, and batteries serve as sources of Cu, Pb, and Cd attributing to the concentration of these elements in the indoor dust [
4,
5]. The high EF of Zn, Cu, Pb, and Ni indicates the presence of anthropogenic sources such as burning fossil fuels, paints, pigments, and metal alloys in household products and building materials [
4]. The moderate EF of As and Cr indicates the enrichment of the metals from varied sources such as wood preservatives, chrome pigments, paints, ink, and anti-corrosive materials [
4].
The As, Cr, Cu, and Fe spatial enrichment maps of the study region (
Figure 6) demonstrates that the western and eastern parts of the Houston metropolitan region were more heavily enriched compared to others. The enrichment of Zn, Mn, and Ni were more pronounced in the western and northwestern regions while the Pb was seen distributed uniformly throughout the study area (
Figure 7). Metal enrichment can be attributed to the various anthropogenic sources such as fossil fuels, industries, along with the individual household facilities and activities [
4].
Health risk assessment revealed that ingestion can be a major pathway for Cd, Cr, and Ni exposure in children (
Table 5). However, ingestion is the primary pathway of children exposure to indoor dust borne metals over inhalation and dermal contact [
39]. Unfortunately, children were found to be more prone to long-term cancer risks from heavy metal exposure than the adults [
39]. There were significant carcinogenic risks (TLCR > 10
−4) from heavy metals in dust samples with Cd, Cr, and Ni being the main contaminants posing carcinogenic risks to children (
Table 5).
In our samples, we observed multiple
Bacillus spp. present.
Bacillus,
Brevibacillus,
Paenibacillus,
Paenisporosarcina, and
Sporosarcina genera are all aerobic, spore-forming bacteria that can withstand industrial pasteurization and form biofilms within pipes and stainless-steel material. These single or multiple-species biofilms become a reservoir of spoilage microorganisms, and a sequence of contamination can be initiated [
40,
41]. Since we observed elevated
Bacillus spp. in our dust samples, foodborne illness can become a major health risk because of the
Bacillus spore-forming bacteria that present spoilage [
42].