Resuspension of biological particles from indoor surfaces: Effects of humidity and air swirl

Effects of surface-attached durations, nutrients, and relative humidity on the resuspension of bacteria during human walking

Resuspension caused by human walking activities is an important source of indoor bioaerosols and has been associated with health effects such as allergies and asthma. However, it is unknown whether inhalation of resuspended bioaerosols is an important exposure pathway for airborne infection. Also, crucial factors influencing the resuspension of settled microbes have not been quantified. In this study, we experimentally investigated the resuspension of culturable bacteria from human-stepping on polyvinyl chloride (PVC) flooring under different conditions. We determined the bacterial resuspension emission factor (ER), a normalized resuspension parameter for the ratio of resuspended mass in the air to the mass of settled particles, for two common bacteria, Escherichia coli and Salmonella enterica. The investigation involved varying factors such as microbial surface-attached durations (0, 1, 2, and 3 days), the absence or presence of nutrients on flooring surfaces, and changes in relative humidity (RH) (35%, 65%, and 85%). The results showed that, in the absence of nutrients, the highest ER values for E. coli and S. enterica were 3.8 × 10-5 ± 5.2 × 10-6 and 5.3 × 10-5 ± 6.0 × 10-6, respectively, associated with surface-attached duration of 0 days. As the surface-attached duration increased from 0 to 3 days, ER values decreased by 92% and 84% for E. coli and S. enterica, respectively. In addition, we observed that ER values decreased with the increasing RH, which is consistent with particle adhesion theory. This research offers valuable insights into microbial resuspension during human walking activities and holds the potential for assisting in the assessment and estimation of risks related to human exposure to bioaerosols.

Indoor air particles in research vessel from Shanghai to Antarctic: Characteristics, influencing factors, and potential controlling pathway

Despite millions of seafarers and passengers staying on ships each year, few studies have been conducted on the indoor air quality inside ship hulls. In this study, we investigated the levels and size distribution of indoor particulate matter during two cruises of the research vessel "Xuelong" from Shanghai to Antarctica. The results showed that the particle size less than 2.5 µm (PM_2.5), and particle size less than 10 µm (PM₁₀) concentrations in different rooms of the ship widely varied. We observed high particulate matter (PM) levels in some of the rooms. The mass concentration distribution was dominated by 1-4 µm particles, which may have been caused by the hygroscopic growth of fine particles. The dominant factors influencing PM concentrations were indoor temperature, relative humidity, and human activity. We quantified contributions of these factors to the levels of indoor particles using a generalized additive model. In clean rooms, the levels of indoor particles were controlled by temperature and relative humidity, whereas in polluted rooms, the levels of indoor particles were mainly influenced by temperature and human activity, which implied that controlling temperature and human activity would efficiently reduce the levels of indoor particles.

Resuspension of particles deposited by nano-enabled consumer sprays: The role of product type, flooring material, and resuspension force

With the development of nanotechnology, an increasing number of nano-enabled consumer products are introduced into the market. The release, deposition, and resuspension of particles from such products could be an important contributor to indoor air pollution and adverse health effects. Our study tested the spray and resuspension of seven nano-enabled consumer products and investigated how flooring material and resuspension force affected the resuspension of particles from these products. Results show that resuspension rates can range from 10^-4 to 5 × 10^-1  h^-1 , depending on the product, flooring material (e.g., carpet and vinyl), and resuspension force (e.g., a walking adult and a moving child; the latter was simulated by a robotic sampler). The resuspension rate from the carpet was statistically significantly higher than that from vinyl flooring, while the resuspension rate by the adult was statiscally significantly higher than that by the robot. In addition, the interaction of investigated factors also played a role in particle resuspension rate. For a subgroup of products based on copper (Cu), silver (Ag), and zinc (Zn) nanomaterials, the resuspension rate reached as high 5 × 10^-1  h^-1 , rates higher than those reported in existing studies with house dust or Arizona Road Dust.

Walking-induced exposure of biological particles simulated by a children robot with different shoes on public floors

Inhalation exposure to the resuspended biological particles from public places can cause adverse effects on human health. In this work, carpet dust samples were first collected from twenty example conference and hotel rooms by a vacuum cleaner. A bipedal robot was then used to simulate children's walking with three different shoes (cotton socks, PVC shoes and EVA shoes) in a hotel room. The particle resuspensions were simultaneously monitored by an aerosol spectrometer. In addition, air samples were also taken using a cyclone liquid impinger operated at 400 L min^-1, and further subjected to gene sequencing analysis. Our results showed that dominant bacterial genera in the carpet dusts included those containing respiratory pathogens such as Staphylococcus, Acinetobacter and Pseudomonas. The bacterial structures in carpet dusts were shown different among the samples from hotel and conference rooms (p < 0.05). Robot-walking resuspended a significant amount of particles from the floors, and different shoes have produced different size and concentration level particles (p < 0.05). Furthermore, walking was observed to resuspend more large particles than smaller ones for the studied range (0.3-10 μm). Robot walking induced increases in airborne Acinetobacter and Pseudomonas in breathing zones that were simulated for children. The results demonstrated that particle resuspension by walking was strongly influenced by particle size, biological species (particle properties), and shoe's sole material. The data from this work provide important information for people especially children aged 1-2 years to protect from resuspension exposure of biological agents when using public floors.

Control of airborne infectious disease in buildings: Evidence and research priorities

The evolution of SARS-CoV-2 virus has resulted in variants likely to be more readily transmitted through respiratory aerosols, underscoring the increased potential for indoor environmental controls to mitigate risk. Use of tight-fitting face masks to trap infectious aerosol in exhaled breath and reduce inhalation exposure to contaminated air is of critical importance for disease control. Administrative controls including the regulation of occupancy and interpersonal spacing are also important, while presenting social and economic challenges. Indoor engineering controls including ventilation, exhaust, air flow control, filtration, and disinfection by germicidal ultraviolet irradiation can reduce reliance on stringent occupancy restrictions. However, the effects of controls-individually and in combination-on reducing infectious aerosol transfer indoors remain to be clearly characterized to the extent needed to support widespread implementation by building operators. We review aerobiologic and epidemiologic evidence of indoor environmental controls against transmission and present a quantitative aerosol transfer scenario illustrating relative differences in exposure at close-interactive, room, and building scales. We identify an overarching need for investment to implement building controls and evaluate their effectiveness on infection in well-characterized and real-world settings, supported by specific, methodological advances. Improved understanding of engineering control effectiveness guides implementation at scale while considering occupant comfort, operational challenges, and energy costs.

Enhanced Capacitive Humidity Sensing Performance at Room Temperature via Hydrogen Bonding of Cyanopyridone-Based Oligothiophene Donor

Cyanopyridone-based oligothiophene donors with both hydrophobic and hydrophilic characters have been evaluated as active layers within simple capacitive devices for humidity sensing at room temperature. Surface studies using atomic force microscopy revealed a self-assembled nanofibrous network with a thin needle-like structure for the terminal hydroxy example (CP6), devoid in the methyl example (CP1). The sensing performance of each sensor was investigated over a broad range of relative humidity levels as a function of capacitance at room temperature. The sensor CP6 demonstrated favourable features such as high sensitivity (12.2 pF/%RH), quick response/recovery (13 s/20.7 s), wide working range of relative humidity (10%–95% RH), low hysteresis (0.57%), outstanding recyclability, and excellent long-term stability. From the results obtained, hydrophilicity and hydrogen bonding appear to play a vital role in enhancing humidity sensing performance, leading to possible new design directions for simple organic semiconductor-based sensors.

Indoor Particulate Matter in Urban Households: Sources, Pathways, Characteristics, Health Effects, and Exposure Mitigation

Particulate matter (PM) is a complex mixture of solid particles and liquid droplets suspended in the air with varying size, shape, and chemical composition which intensifies significant concern due to severe health effects. Based on the well-established human health effects of outdoor PM, health-based standards for outdoor air have been promoted (e.g., the National Ambient Air Quality Standards formulated by the U.S.). Due to the exchange of indoor and outdoor air, the chemical composition of indoor particulate matter is related to the sources and components of outdoor PM. However, PM in the indoor environment has the potential to exceed outdoor PM levels. Indoor PM includes particles of outdoor origin that drift indoors and particles that originate from indoor activities, which include cooking, fireplaces, smoking, fuel combustion for heating, human activities, and burning incense. Indoor PM can be enriched with inorganic and organic contaminants, including toxic heavy metals and carcinogenic volatile organic compounds. As a potential health hazard, indoor exposure to PM has received increased attention in recent years because people spend most of their time indoors. In addition, as the quantity, quality, and scope of the research have expanded, it is necessary to conduct a systematic review of indoor PM. This review discusses the sources, pathways, characteristics, health effects, and exposure mitigation of indoor PM. Practical solutions and steps to reduce exposure to indoor PM are also discussed.

Human Emissions of Size-Resolved Fluorescent Aerosol Particles: Influence of Personal and Environmental Factors

Human emissions of fluorescent aerosol particles (FAPs) can influence the biological burden of indoor air. Yet, quantification of FAP emissions from human beings remains limited, along with a poor understanding of the underlying emission mechanisms. To reduce the knowledge gap, we characterized human emissions of size-segregated FAPs (1-10 μm) and total particles in a climate chamber with low-background particle levels. We probed the influence of several personal factors (clothing coverage and age) and environmental parameters (level of ozone, air temperature, and relative humidity) on particle emissions from human volunteers. A material-balance model showed that the mean emission rate ranged 5.3-16 × 10⁶ fluorescent particles per person-h (0.30-1.2 mg per person-h), with a dominant size mode within 3-5 μm. Volunteers wearing long-sleeve shirts and pants produced 40% more FAPs relative to those wearing t-shirts and shorts. Particle emissions varied across the age groups: seniors (average age 70.5 years) generated 50% fewer FAPs compared to young adults (25.0 years) and teenagers (13.8 years). While we did not observe a measurable influence of ozone (0 vs 40 ppb) on human FAP emissions, there was a strong influence of relative humidity (34 vs 62%), with FAP emissions decreasing by 30-60% at higher humidity.

Falling bacterial communities from the atmosphere

BACKGROUND

Bacteria emitted into the atmosphere eventually settle to the pedosphere via sedimentation (dry deposition) or precipitation (wet deposition), constituting a part of the global cycling of substances on Earth, including the water cycle. In this study, we aim to investigate the taxonomic compositions and flux densities of bacterial deposition, for which little is known regarding the relative contributions of each mode of atmospheric deposition, the taxonomic structures and memberships, and the aerodynamic properties in the atmosphere.

RESULTS

Precipitation was found to dominate atmospheric bacterial deposition, contributing to 95% of the total flux density at our sampling site in Korea, while bacterial communities in precipitation were significantly different from those in sedimentation, in terms of both their structures and memberships. Large aerodynamic diameters of atmospheric bacteria were observed, with an annual mean of 8.84 μm, which appears to be related to their large sedimentation velocities, with an annual mean of 1.72 cm s^- 1 for all bacterial taxa combined. The observed mean sedimentation velocity for atmospheric bacteria was larger than the previously reported mean sedimentation velocities for fungi and plants.

CONCLUSIONS

Large aerodynamic diameters of atmospheric bacteria, which are likely due to the aggregation and/or attachment to other larger particles, are thought to contribute to large sedimentation velocities, high efficiencies as cloud nuclei, and large amounts of precipitation of atmospheric bacteria. Moreover, the different microbiotas between precipitation and sedimentation might indicate specific bacterial involvement and/or selective bacterial growth in clouds. Overall, our findings add novel insight into how bacteria participate in atmospheric processes and material circulations, including hydrological circulation, on Earth.

Sensitisation of severe asthma in different occupations: A multicentre study in China

OBJECTIVE

This was the first multicentre study assessing the sensitisation patterns in patients with severe asthma by different occupations as well as different regions of China, and aimed to provide evidence for the prevention of sensitised patients with severe asthma.

METHODS

A total of 676 adults were included and assigned to three cohorts. Cohort A was comprised of severe asthma patients without smoking (n = 454), cohort B comprised severe asthmatic patients who were ex-smokers (n = 149), and cohort C comprised non-asthma individuals (n = 73). All study subjects underwent testing of serum sIgE level of house dusts mix (hx2), foods mix (fx5), moulds mix (mx2), tree pollen mix (tx4) and weed pollen mix (wx5) simultaneously.

RESULTS

Overall, the positive rates of hx2, fx5, mx2, tx4 and wx5 were 37.6%, 11.9%, 16.6%, 9.0% and 9.5% in severe asthmatic patients, respectively. Comparing cohort A with cohort B, the hx2 positive rates in cohort A were higher in fulltime homemaker (30.0% vs. 0.0%) and medical staff (40.0% vs. 0.0%, P < 0.05). However, the mx2 positive rates in cohort B were higher in businessman (30.0% vs. 7.0%) and the tx4 positive rates in cohort B were higher in drivers (35.7% vs. 0.0%, P < 0.05). The optimal scale analysis showed that severe asthmatic patients who work indoor were more prone to be allergic to house dusts (Cronbach's alpha = 76.4%).

CONCLUSION

Patients with severe asthma in different occupations exhibited different sensitisation patterns. Avoid being contact with house dust and application of indoor air purifier should be considered for severe asthma patients.

Airborne Bacteria in Outdoor Air and Air of Mechanically Ventilated Buildings at City Scale in Hong Kong across Seasons

Studies of the indoor airborne microbiome have mostly been confined to a single location and time point. Here, we characterized, over the course of a year, the geographic variation, building-function dependence, and dispersal characteristics of indoor and outdoor airborne microbiomes (bacterial members only) of eight mechanically ventilated commercial buildings. Based on the Sloan neutral model, airborne microbiomes were randomly dispersed in the respective indoor and outdoor environments and between the two environments during each season. The dominant taxa in the indoor and outdoor environments showed minor variations at each location among seasons. The airborne microbiomes displayed weak seasonality for both indoor and outdoor environments, while a weak geographic variation was found only for the indoor environments. Source tracking results show that outdoor air and occupant skin were major contributors to the indoor airborne microbiomes, but the extent of the contribution from each source varied within and among buildings over the seasons, which suggests variations in local building use. Based on 32 cases of indoor airborne microbiome data, we determined that the indoor/outdoor (I/O) ratio of PM_2.5 was not a robust indicator of the sources found indoors. Alternatively, the indoor concentration of carbon dioxide was more closely correlated with the major sources of the indoor airborne microbiome in mechanically ventilated environments.

Microbial emission levels and diversities from different land use types

Bioaerosol particles, originated from many different earth ground sources, have unique health impacts, including respiratory infections, allergic reactions, and toxic effects. Here, we applied a portable high-flow sampler HighBioTrap to collect and investigate bioaerosol emissions from 13 different land types (forest, wetland, lake, bare soil, cropland, wastewater treatment facility, street, livestock farm, smeltery and garden) that are heavily or less affected by humans. Plate cultivation, real-time quantitative PCR analysis (q-PCR) and high-throughput gene sequencing analysis were used to characterize bacterial and fungal levels as well as their community structures emitted from different land use types. Results showed that there were statistically significant differences in biological emission levels (up to 100-fold difference) and diversity among different land use types. Cropland, sewage plant street and smeltery heavily affected by human activities were found to exhibit higher bioaerosol emission levels, with Massilia genus detected as the dominant species. In contrast, some land types (lakes, forests, gardens, and wetland) less affected by humans were found to emit lower bioaerosol levels but with higher culturability, e.g., up to 16% for wetland. In addition, the microbiological structures of these land-use types usually had higher species richness and diversity, yet different dominant species. For some land types such as streets in Beijing, the microbial community appeared to be skewed with an over 80% relative abundance of a specific dominant species such as Massilia. Other detected dominant species also included Acinetobacter and Brevundimonas for street, and Sphingomonas for wetland. For fungal community, Naganishia, Alternaria, Penicillium, and Aureobasidium were detected to be most abundant. RDA analysis showed metals and ions could to some extent affect the microbial community structures. This work highlights that the human activities could substantially affect the airborne microbiota, which in turn could affect local human health and ecosystems. On the other hand, the results here provide important references for quantitatively estimating the microbial emissions from the earth into the atmosphere.

An experiment and numerical study of resuspension of fungal spore particles from HVAC ducts

Fungal spore resuspension on the surfaces of the heating, ventilation and air-conditioning (HVAC) ducts have been verified as one of the most important factors causing indoor biological pollution. To effectively control indoor bioaerosols pollution, it is essential to understand the resuspension characteristics of fungal spores in HVAC ducts. Therefore, this study aims to investigate the movement behavior of particles and further the variation of particle resuspension with HVAC operation mode. Based on the experimental and numerical study, this study specifically investigated the micro-movement behavior of particles and impact of particle size (1, 4, 7 and 10 μm), air temperature (9, 15 and 30 °C), relative humidity (20, 50 and 80%), duct surface roughness (0.5 and 50 μm) and air velocity (0.3, 0.9, 1.5 and 2.5 m/s) on the short-term resuspension of particle in horizontal HVAC ducts. Results indicate that spore particles were dominantly rolling off rather than sliding or being lifted into the air. Compared with larger particles, smaller ones were more sensitive to the wall roughness. The resuspension rate of spores was mainly affected by airflow velocity and particle size, where the resuspension rate of particles increased by up to six times with the increase of airflow velocity from 0.9 to 2.5 m/s. In comparison, either air temperature or relative humidity made negligible difference to particle resuspension rate. Overall, this study provides the knowledge of fungal spore resuspension in HVAC ducts, laying the foundation for effectively controlling the indoor biological pollution.

Ten questions 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 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.

Resuspension of settled atmospheric particulate matter on plant leaves determined by wind and leaf surface characteristics

Atmospheric particulate matter (APM) is temporarily settled on the leaf surface of plants and will return to the air via the resuspension process under certain meteorological conditions. How leaf surface characteristics affect the resuspension of settled APM on the leaf surface has been rarely studied. Therefore, the resuspension of APM after settling on plant leaves was analyzed using four common urban greening species, including Prunus triloba, Platanus acerifolia, Lonicera maackii, and Cercis chinensis. The results show that the leaf hair density has a significantly positive correlation with the maximum particulate matter (PM) retention and natural PM retention (p < 0.05). Under the same wind speed, the proportions of the resuspended PM that settled on the leaf surfaces of the four plant species increase with the wind blowing time. During the same wind blowing time, the resuspension rate of the settled PM on leaf surfaces of P. triloba, P. acerifolia, and L. maackii increase with the wind speed. The leaf hair and stomatal density is negatively correlated to the resuspension rate of PM under the wind speed of 1 m s^-1 (p < 0.05), and the stomatal density is also negatively correlated to the resuspension rate of PM under the wind speed of 5 m s^-1 for 10 min or 20 min (p < 0.05). However, as the wind speed further increase, the leaf characteristics are no longer correlated to the resuspension rate of PM (p > 0.05). These results indicate that when the wind force (wind speed + wind blowing time) is small, the stomatal density and leaf hair density have a significant effect on APM resuspension. When the wind force is large, the influence of leaf surface structure on APM resuspension becomes less profound. APM resuspension is comprehensively affected by the external wind and the leaf surface characteristics, and these two factors jointly determine the fate of the PM after it settles on leaves.

Biological Particles in the Indoor Environment

Airborne microorganisms are very difficult to assess accurately under field conditions owing to differences in the sample collection efficiency of the selected sampler and variations in DNA extraction efficiencies. Consequently, bioaerosol abundance and biodiversity can be underestimated, making it more difficult to link specific bioaerosol components to diseases and human health risk. Owing to the low biomass in air samples, it remains a challenge to obtain a representative microbiological sample to recover sufficient DNA for downstream analyses. Improved sampling methods are particularly crucial, especially for investigating viral communities, owing to the extremely low biomass of viral particles in the air compared with other environments. Without detailed information about sampling, characterization and enumeration techniques, interpretation of exposure level is very difficult. Despite this, bioaerosol research has been enhanced by molecular tools, especially next-generation sequencing approaches that have allowed faster and more detailed characterization of air samples.

The influence of relative humidity and ground material on indoor walking-induced particle resuspension

The resuspension of indoor particulate matters caused by people indoor walking could affect indoor air quality and human health. Therefore, it is particularly important to study the resuspension rules of the particulate matters in different indoor environments. The influence of the ground material and the relative humidity on resuspension of the particulate matters were investigated under three kinds of ground materials and three different relative humidity. Results showed that different relative humidity and different ground materials had different effects on the mass concentration of the particulate matters. In addition, different particle sizes had diverse influence on the mass concentration. Compared with low-level loop pile carpet and shaggy carpet, hardwood floor was more conductive to human health which was less likely to cause the resuspension of the particulate matters. At the same time, relative humidity had a great influence on the resuspension of the particulate matters. With the increase of relative humidity, the resuspension rate of fine particulate matters decreased.

Effect of Non-Coal Heating and Traditional Heating on Indoor Environment of Rural Houses in Tianjin

In order to understand the effect of the non-coal heating and the traditional coal-fired heating on the indoor environment of the rural houses, the humidity environment and indoor air quality in several households were investigated during the heating period in Beichen District and Wuqing District of Tianjin, China. The results showed that the indoor average temperature for the heating by the electricity and the natural gas was higher than that by the traditional coal fire. The indoor relative humidity for the heating by the electricity and the natural gas was lower than that by the traditional coal fire. The indoor air quality (IAQ) for the heating by the electricity and the natural gas was better than that by the traditional coal fire. For traditional coal-fire heating households, the indoor pollutant emission (CO emission) by using the clean coal was lower than that by using the raw coal. The indoor ventilation rate which was an important parameter for the indoor air quality was generally poor in winter. The total volatile organic compounds (TVOC) emission in the indoors of the coal-fired heating households was generally higher than that of the non-coaled heating households.

Research Advances in Microfiber Humidity Sensors

An overview of the numerous latest research in microfiber humidity sensors is carried out with a specific focus on measurement methods, humidity sensitive materials, probe structures, and sensing properties of different sensors. First, five mainstream measurement structures, including taper, fiber grating, coupler, resonator, and interferometer are reviewed. It is concluded that these measurement structures sense the physicochemical property variations of microfibers or sensitive films and exhibit the change of optical signal when exposed to environment. Second, the basic preparation methods, humidity-sensing properties, and their advantages and disadvantages as humidity sensitive material are addressed. Then, the advantages and disadvantages of all the above sensing structures are also discussed and compared. Finally, the main existing problems and potential solutions of microfiber humidity sensors are pointed out.

Do Carpets Impair Indoor Air Quality and Cause Adverse Health Outcomes: A Review

Several earlier studies have shown the presence of more dust and allergens in carpets compared with non-carpeted floors. At the same time, adverse effects of carpeted floors on perceived indoor air quality as well as worsening of symptoms in individuals with asthma and allergies were reported. Avoiding extensive carpet use in offices, schools, kindergartens and bedrooms has therefore been recommended by several health authorities. More recently, carpet producers have argued that former assessments were obsolete and that modern rugs are unproblematic, even for those with asthma and allergies. To investigate whether the recommendation to be cautious with the use of carpets is still valid, or whether there are new data supporting that carpet flooring do not present a problem for indoor air quality and health, we have reviewed the literature on this matter. We have not found updated peer reviewed evidence that carpeted floor is unproblematic for the indoor environment. On the contrary, also more recent data support that carpets may act as a repository for pollutants which may become resuspended upon activity in the carpeted area. Also, the use of carpets is still linked to perception of reduced indoor air quality as well as adverse health effects as previously reported. To our knowledge, there are no publications that report on deposition of pollutants and adverse health outcomes associated with modern rugs. However, due to the three-dimensional structure of carpets, any carpet will to some extent act like a sink. Thus, continued caution should still be exercised when considering the use of wall-to-wall carpeted floors in schools, kindergartens and offices, as well as in children’s bedrooms unless special needs indicate that carpets are preferable.