Construction Period Research Articles

Indoor radon trends with building code change in two Canadian cities

Radon studies were conducted in two Canadian cities, in Halifax, Nova Scotia, and Winnipeg, Manitoba, to evaluate trends in indoor radon before and after the 2010 National Building Code of Canada was adopted into the legally binding provincial building codes in 2011. Participants were recruited in neighbourhoods characterized by newer housing developments. A postcard campaign in each city offered free radon testing to every house in the target areas, and free testing kits were mailed to study participants. Indoor radon measurements and house questionnaires were completed by 225 eligible participants in Halifax and 171 eligible participants in Winnipeg, using alpha-track radon detectors deployed for three months during the heating season in 2021–2022. Multivariate logistic regression analyses were conducted to evaluate the association between indoor radon and the period of construction, the area in contact with the ground, the number of storeys, the type of heating system, the water source, and the type of ventilation. These analyses were focussed on the detached study houses because the majority of the participants lived in a detached house, and significant associations were identified for the period of construction and the floor where the radon detector was located. An odds ratio of 1.91 (1.04–3.50) for the detached Halifax study houses built after 2011 was associated with having a higher than geometric mean radon concentration (p = 0.033), nearly double the likelihood. There was no evidence of significant change in the indoor radon after 2011 in the detached Winnipeg study houses. A lower likelihood – almost half - for measurement conducted on a main/upper floor compared to in the basement was associated with a radon concentration above the respective geometric mean for each city: an odds ratio of 0.48 (0.27–0.86) for the detached Halifax study houses (p = 0.012), and of 0.45 (0.32–0.64) for the detached Winnipeg study houses (p = 0.022). Radon is the second most important cause of lung cancer, after smoking, and the results of this study support strengthening the radon preventive measures required in new low-rise housing to reduce the associated lung cancer burden in Canada.

Fluidized solidified soil using construction slurry improved by fly ash and slag: preparation, mechanical property, and microstructure

Abstract Fluidized solidified soil (FSS) is a cement-based engineering matergood working performance and mechanical properties. Based on fixed cement and desulphurisation gypsum (DG), fly ash (FA) and ground granulated blast furnace slag (GGBS) were added as admixtures to the construction slurry to prepare three types of FSS: namely cement-GGBS-DG FSS (CGD-FSS), cement-FA-GGBS-DG FSS (CFGD-FSS), and cement-FA-DG FSS (CFD-FSS). Considering 7 d, 14 d, and 28 d three curing times, compressive, flexural, scanning electron microscopy (SEM), and x-ray diffraction (XRD) analyses were conducted to explore the time-dependent mechanical properties and microscopic characterisation of FSS. The mechanical test showed that CFGD-FSS doped with FA and GGBS had better fluidity, compressive strength, and flexural strength than CGD-FSS doped with FA alone and CFD-FSS doped with GGBS. The CFGD-FSS specimen with a cement:FA:GGBS:DG ratio of 30: 10: 40: 20 in the curing agent had the best mechanical properties, i.e., the CFGD01 specimens. It has fluidity of 189 mm, compressive strength of 671 kPa, and flexural strength of 221 kPa with a 28d curing time, which can meet the working requirements of FSS for filling narrow engineering spaces. And compared with other specimens, it has the shortest setting time, which can effectively shorten the construction period. Microscopic analysis showed that a large number of hydration products, such as calcium silicate hydrate, calcium aluminate hydrate, and ettringite (Aft), were well-formed in the FSS, resulting in good mechanical properties, especially for the CFGD-01 specimens. Finally, two empirical models were established to describe the compressive strength–porosity and flexural strength–porosity relationships. Moreover, the investigated data agreed well with the modelling results.

Data Classification System for Economic Analysis after the Implementation of Separate Order Construction in the Firefighting System Installation Business

Data on the performance of the firefighting system installation business are scarce compared with other construction projects, such as electricity and information and communication projects, where separate order construction was previously practiced. This study aimed to obtain data on fire facility construction contracts by analyzing the current data, deriving improvement plans, and reviewing empirical studies on separate order construction from various construction projects, both domestically and internationally. An analysis of the data from Statistics Korea and the Public Procurement Service revealed an annual increase in the number and amount of firefighting system installation projects. In addition, the scale of the construction could be identified using separate order construction data. This study highlights cost reduction, shortened construction delay periods, and increased construction speed as the key economic review items. Based on these findings, a data classification system for the construction of firefighting facilities was proposed and categorized according to the project size, ordering organization, contractor, type of construction, construction period, and contract method.

The Impact of MRT Line-6 Construction Work on The Environment, Road Users, and Traffic During the Construction Period

The Impact of MRT Line-6 Construction Work on The Environment, Road Users, and Traffic During the Construction Period

Contingent Attachment: Long-term Lived Experience in the Redevelopment of a Third Front Neighbourhood

Abstract Employing the concept of “contingent attachment” as its key point, this paper investigates the transformation of residents’ neighbourhood attachment during the redevelopment of a neighbourhood originally established during the Third Front construction period. By framing neighbourhood attachment as contingent, this paper seeks to highlight that the different directions of neighbourhood attachment hinge upon several factors: the entities mobilizing the narrative, the varying treatment of residents across different phases of the relocation, and residents’ perceptions regarding the changes brought about by the relocation process. Special attention within this paper is paid towards the long-term lived experiences of residents since the onset of the Third Front construction to show how these experiences are woven into diverse narratives associated with residents’ neighbourhood attachments.

Distribution of Microorganisms and Antibiotic Resistance Genes in Production Wastewater During Pumped Storage Power Station Construction

The construction period of pumped storage power stations (PSPS) generates amounts of production wastewater, which may contain pathogenic bacteria and antibiotic resistance genes (ARGs) in these bacteria, potentially posing environmental and health risks. This study used the metagenome approach to analyze the distribution of microorganisms, ARGs and their correlation with water quality indicators in wastewater collected from two typical PSPSs. Coagulation system wastewater exhibits strong alkalinity (11.88), and aggregate system wastewater has high suspended solids (SS, 8 × 104 mg/L), resulting in lower richness and diversity of bacterial communities. Serpentinimonas, a kind of alkaliphilic bacteria, had the highest relative abundance (48.58–99.7%). The ARG subtypes obtained conferred wastewater resistance to tetracycline, macrolide, fluoroquinolone and so on, but wastewater treatment has limited removal effect on ARGs. The results indicate that resistant bacteria and resistance genes can still be present and distributed under highly alkaline conditions, and the removal efficiency of ARGs by wastewater treatment in PSPS is limited. Attention should be given to the environmental and health risks posed by production wastewater, thereby providing a theoretical basis for the sustainable development of the PSPS industry.

An Experimental Study on the Force Characteristics and Stability of an FRP Pipe Floating in Mud

Based on the occurrence of floating and fracture accidents of FRP pipes on muddy seabeds in practical engineering combined with the water intake and drainage project of the 2 × 660 MW coal-fired power plant in the second phase along the coast of Vietnam, a model test study was carried out. The laboratory simulated the FRP pipe conditions during the construction and operation periods by configuring different densities of silt. The experiment results obtained are as follows: the FRP pipes floated up in the mud during the construction period; the thickness of sediment deposition when the FRP pipes are lifted up, as well as the entire lifting process time and maximum force, are all within 2 h and 70 kN/m; after adopting a cover layer design section during the operation period, the FRP pipes in the silt remain stable while obtaining a maximum force of 15 kN/m; and based on the experimental results, an empirical formula for the stress on FRP pipes and was derived and measures for FRP pipe stability were proposed. The research results not only solve practical problems in water intake and drainage engineering but also provide technical support for the promotion and application of FRP pipes.

Emission Risk and Inhibition Technology of Asphalt Fume from Crumb Rubber Modified Asphalt

Crumb rubber-modified asphalt mixtures have been proven to have extensive utilization value in road engineering. However, the rubber releases more fumes during the construction period, which causes severe harm to human health and the environment. This research focused on the emission risk of asphalt fume from crumb rubber-modified asphalt, and then the inhibition technology was also optimized. Firstly, the emission behavior and the hazardous evaluation of the asphalt fume from crumb rubber-modified asphalt were investigated. Then, the characteristics of the inhibition materials were evaluated. Finally, the reduction in the emission of inhibited crumb rubber-modified asphalt fume was identified, and the optimized formula was determined based on the inhibition effect, rheological properties, and cost. The results indicate that crumb rubber-modified asphalts release more fume components with an increment in the temperature and crumb rubber content. Desulfurized rubber reduces the release of H2S and NO. Benzene compounds, including paraxylene, toluene, and benzene, are the most released pollutants that harm human health, especially DS CRA 20% and CRA 50%. Kaolin powder and expanded graphite have a sufficient pore structure and volume, the addition of which reduces the release of pollutants while possibly promoting the release of NO and H2S. Their addition also has a significant control effect on the release of particulate matter at 170 °C and 185 °C. With the consideration of emissions, rheological properties, and cost, CRA 40%-EG2%-KL2% was determined as the optimization formula. This research is helpful to the application of crumb rubber-modified asphalt in road construction and maintenance.

Study on the Settlement and Deformation Characteristics of Surrounding Rock of Loess Tunnel

There is a water culvert with masonry structure in the mountain of Hanguguan Tunnel. Soil deformation caused by tunnel excavation leads to channel cracking. On the one hand, channel water infiltration leads to loess tunnel collapse, which can easily cause engineering accidents and delay the construction period. On the other hand, it will affect the channel water supply and cause social problems. In this paper, the spatial deformation of the tunnel excavation process in the loess layer is revealed through the settlement of the vault in the tunnel, the monitoring of the convergence of the perimeter, and the force monitoring of the lining support structure of the Hanguguan tunnel, and the influence of the tunnel excavation deformation on the culvert is revealed through numerical calculation. The results show that the arch at the tunnel exit has the largest settlement deformation, the larger cumulative settlement, and the faster settlement rate, and the deformation of the culvert is small, only a millimeter. Compared with the monitoring data, the numerical simulation of tunnel excavation and the vertical settlement are basically consistent with the actual situation.

Climate Change and Building Renovation: The Impact of Historical, Current, and Future Climatic Files on a School in Central Italy

Climate change significantly affects the operating environment of buildings. These changes impact both energy efficiency and occupants’ comfort and remain crucial even in building restoration, where design decisions typically rely on historical data, yet performance depends on anticipated future scenarios. The present work evaluates the impact of different climate datasets on dynamic energy simulations for an educational building in Central Italy, focusing on estimating heating demands across historical, current, and future climatic scenarios. The assessment considers both the building’s current state and potential energy-efficient retrofits. Initially, various meteorological datasets, including measured and model-generated data, are selected to predict key weather parameters. The analysis reveals the potential and limitations of regional climate models (RCMs) in estimating these variables, with the MM5 dataset emerging as the most reliable. Subsequently, the energy performance of the reference building and its vulnerability to climate change are assessed. Our results show significant differences in energy demand based on construction periods, with the oldest section consuming 29% to 54% more energy monthly than the newer sections. Moreover, using non-representative climatic files can lead to prediction errors of up to 199%. Finally, the building’s energy behaviour is analysed under future climate conditions by generating typical meteorological years (TMYs) for 2030, 2050, and 2070. This analysis evaluates the energy requirements for both existing and retrofitted building configurations. The findings confirm that retrofit interventions with high-performance insulation and upgraded windows significantly enhance the building’s energy efficiency and resilience to future climate conditions, leading to annual energy savings of 50% to 57%.

Coupling dynamic characteristics of installation vessel - Multi buckets jacket foundation lowering system

Precise control of the splash zone lowering process of the multi-bucket jacket foundation for an offshore wind turbine can save time during the entire construction period, thereby reducing construction and installation costs. This paper focuses on the “installation vessel - multi buckets jacket foundation” coupled dynamic system during the lowering process of the multi-bucket jacket foundation. Taking the three-bucket jacket foundation (TBJF) and the four-bucket jacket foundation (FBJF) as the research objects, four different sling layout types were proposed. The influence of the slings layout types on the motion response of the installation vessel and foundation and the slings' tension in this system were comparatively studied through numerical simulation. Research shows that through the lowering of Type 1 conditions, that is, each chord corresponds to a vertical sling and is directly connected to the crane's lifting point, the motion performance of the foundation and installation vessel can be reduced, ensuring the smooth progress of the lowering process of the structure. Under the same conditions, the FBJF has greater motion response due to the large wave force bearing area and large water plane area. Under different conditions, the DAF value and failure factor of the slings of the two structures during the lowering process all meet the specification requirements.

Methodology for selecting optimal wire grade and cross-section based on an integrated technical and economic criterion

The aim was to develop a methodology for selecting an optimal wire grade and cross-section for overhead power lines with a voltage of above 1 kV under intensive development of power grid systems. This aim was solved using the authors’ previously developed methods: selection of an optimal wire grade based on hierarchy analysis and selection of an optimal wire cross-section by optimizing the specific discounted costs during the entire period of construction and operation of overhead power lines. In the present study, these methods are integrated into a single methodology. It is proposed to implement wire inspections prior to the stage of technical and economic calculations, since the necessary data has already been taken into account during the selection of a wire grade and its cross-section. The proposed methodology was tested on a typical example of reconstruction of the Zapadnaya– Davydovka 110 kV overhead line, which creates limitations in the power supply of Primorsky Krai due to insufficient wire capacity and its operation beyond the normative period. SENILEK AT3/C 150/24 wire was selected as a solution for the example under consideration. The application of this wire grade allows not only the overhead line capacity to be increased by 151% without replacing the existing supports, but also active and reactive power losses in the electric network to be reduced by 18.9 and 2.5%, respectively. The proposed methodology enables selection of an optimal grade and cross-section of any wire design, taking the dynamically changing operational conditions of power grid systems into account. The solutions found by the proposed methodology may contribute to a more efficient operation of power lines due to increasing their transmission capacity, reducing the number of used supports, replacing the line wire without replacing supports, decreasing ice formation on wires, and reducing power losses.

A systematic literature review on occupational accident factors in the rail construction industry: lessons learned from a quarter-century of studies globally.

The rail construction industry is notable for its large scale, substantial investment, extensive stakeholders involvement, long construction period, and intricate operation and technology. This industry is among the most dangerous due to the highest number of occupational accident cases worldwide. Therefore, it is crucial to analyse and identify the existing literature on occupational accident factors in rail construction. To address the research aim, the study identified the factors that contribute to occupational accidents using systematic review methodology. This systematic literature review adheres to the rigorous Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) 2020 statement. Relevant publications from the past 25 years were retrieved from Scopus, Web of Science (WoS), and Science Direct electronic databases. Through a meticulous review of 43 selected publications, five accident factor themes were discovered: worker, workplace, materials and equipment, organizational, and environmental influences. The detailed analysis of these themes has led to the identification of 19 specific sub-factors within these categories, providing a granular understanding of the intricate elements contributing to accidents. This study offers a foundational understanding of accident factors in the rail construction industry, paving the way for targeted OSH interventions aimed at preventing occupational accidents in the future.

Research and Application of Drilling and Completion Technologies on Large-size Injection-production Well

Abstract The large-size injection-production well can increase the injection-production capacity to more than 3 times, compared with the conventional-size one. However, it had not been popularized and promoted for many years since it had the technical difficulties of long construction period, high engineering risk and large investment in a single well. The Liaohe double-6 gas storage planned to deploy 3 large-size horizontal wells, aiming at further improving injection-production capacity to satisfy peak shaving demand. To overcome the technical difficulties of large-size well, the drilling scheme was optimized according to geological characteristics of double-6 block in the paper. It included wellbore structure design, wellbore diameter control technology and hole cleaning technology. The optimized drilling scheme can not only improve the wellbore integrity, but also reduce the risk of engineering accidents and shorten the drilling period greatly. Besides, the cementing technology was optimized as well from the aspects of hierarchical cementing and cement slurry design, which can be used to solve the problems of high pressure at the bottom of well and large leaking risk of cement. On the basis, the above drilling and cementing technologies were applied to develop three large-size wells, and good practical results were achieved. The average completion period of the three well was 104 days, 36 days shorter than the planned period, which realized the applying effects that large-size wells can be designed, constructed, and produced in the same year. Moreover, the ultrasonic evaluation results showed that the average qualified rates of the three wells in technical casing was 92.8%, which was much higher than the industry standard. Besides, the test results showed that the injection and production volume of each large-size well in the paper can exceed 3 million cubic meters, solving the problem of insufficient injection-production capacity in the months with high peak shaving demand. Above all, the drilling and cementing technologies of large-size well had been optimized in the paper, which successfully reduced the well completion period, and improve the cementing quality and the safety of drilling construction. The researches can not only ensure the project quality of large-size well in Liaohe double-6 gas storage, but also provide technical support for promoting the drilling and completion engineering of large-size well in other domestic and foreign gas storages.

Research and Application of Chemical Water Plugging Technology with High Salt Resistant Gel and Immobile String

Abstract The salinity of formation water in Gasi N1-N2 reservoir is 18 × 104 mg/L, in which calcium and magnesium ions are 6300 mg/L. The conventional gel used for profile control and water plugging has poor salt resistance and is easy to dehydrate under the erosion of formation water; At the same time, the chemical water plugging of oil wells in the past needs to trip down the downhole string for many times, and the construction period is long. In order to solve the former problem, an anti-high salt organic gel suitable for Gasi N1-N21 reservoir is synthesized. The formula is 0.4 wt% SD-3000 + 0.1-0.2 wt% urotropine + 0.05 wt% resorcinol + 0.4 wt% additive. The viscosity of the gel is more than 6000 mPa.s after gelation, and no obvious dehydration phenomenon is found after aging for five months in the formation water environment of Gasi N1-N21 reservoir at 65 °C. The technology of chemical water plugging without moving the string has been innovatively developed, which does not require the operation of tubing and pump during construction. In Gasi N1-N21 reservoir, anti-high salt gel was used to carry out three well-times of chemical water plugging tests without pulling the string, which were all successful, saving 44% of the cost of measures and shortening 78% of the construction period. After water plugging, water production was obviously inhibited, and the average moisture content was reduced from 95% to 45%. The average daily oil increment of a single well was 1.8 tons, and the cumulative oil increment was 1700 tons. The results show that the chemical water plugging technology without pulling string has broad application prospects in Gasi N1-N21 reservoir in Qinghai Oilfield and similar reservoirs in China.

Stability Analysis of Surface Wellhead Retention in Offshore Exploration Wells

Abstract When offshore exploration wells are carried out, a narrow operating window and prolonged construction period may be encountered, which may lead to the evacuation of the drilling platform. The conductor will be left freestanding in the sea water, posing a great risk to wellhead stability. In this paper, based on finite element simulation of offshore exploratory well freestanding conductors, the stability analysis method is built and evaluation of a well is carried out. It is concluded that in 1 year return period and sea conditions by hanging load return period of 10 years of sea condition, the stability all meet the conditions, but there in 10 years return period of sea state by hanging load and 50 years return period of sea condition did not meet the requirements of stability. In cases where stability requirements are not met, alternative means of retaining the wellhead are recommended. This study has certain guiding significance for the safe and efficient operation of offshore exploration wells.

Research and Application of Plugging Drilling and Completion Technology for Difficult Old Well in Changchun Gas Storage

Abstract Changchun Gas Storage has been included in the 14th Five-Year Plan for the development of petroleum and natural gas in Jilin Province. Its smooth construction and safe operation are crucial to ensure the stable supply of natural gas in Changchun. Well X106F located in the construction area was abandoned in 1988 due to drilling tool falling and multiple salvage failures. In order to ensure the sealing and integrity of the gas storage, it is necessary to effectively plug the drilling cover. In view of the problems of low well history accuracy, poor well wall stability and difficulty in cementing large boreholes, open hole drilling, trajectory design and control, film-forming and wall-fixing drilling fluid and prestressed cementing technologies are applied. By optimizing drilling tools, optimizing drilling parameters, designing the three-opening well structure and “five-section” profile, the plugging well construction requirements are met. The first and second open drilling along the old well borehole, 1313m deep on the first try to find top of the fish, after cementing plug, side tracking to 50m below the top of the fish and then cementing;for third spud control trajectory to accompanied the old well, according to the results of passive magnetic guidance, real-time adjustment of the trajectory scheme and construction plan, finally successfully collided with the old well at 1893m deep, after cementing the drilling plug, perforating and extruding cement, the cement job quality is good. The whole well construction period of 100 days, creating the fastest record of the old well plugging without trajectory of the fish type, providing technical support for the construction of Changchun gas storage, and providing experience for the same type of old well plugging and complex well treatment in China.

A multi-scale method for the study of deep drying of ethylene with 3A zeolite

Coal-based ethanol to ethylene (ETE) is an emerging production pathway with the advantages of a short construction period, high product purity, and few by-products. N2 is commonly used in industry to regenerate zeolites during the ETE process, with the problems of excessive material consumption, energy consumption and environmental pollution. To solve this problem, this paper proposed a method of using the ethylene product as regeneration gas to regenerate the zeolite and recover the desorbed gas. In this work, we adopted a multi-scale simulation method combining microscale apparent diffusivity and macroscale mass transfer models. Reactive force field and molecular dynamics (ReaxFF-MD) were used to obtain an equation for the diffusion coefficient versus temperature. The start-up of the twin-tower temperature and pressure swing adsorption (TPSA) process was simulated using Computational Fluid Dynamics (CFD). The TPSA process achieved an outlet water content below 5 ppm, fulfilling the process specifications.

Traffic Impact Analysis Due to Construction of Special Eye Hospital Padang Eye Center

Padang Eye Center is one of the hospitals under construction on Jalan Khatib Sulaiman, Lolong Belanti, North Padang District, Padang City, West Sumatra. The construction of the hospital will certainly have an impact on traffic on Jalan Khatib Sulaiman. The purpose of conducting andalalin is to analyze the amount of travel demand that occurs at the Padang Eye Center Hospital in pre-construction, construction period, operational period, and post 5 years of construction, as well as seeing the impacts that occur and how alternative solutions are carried out. The parameters used are based on the Regulation of the Minister of Transportation of the Republic of Indonesia Number 17 of 2021 concerning the implementation of andalalin, the Indonesian Road Capacity Guidelines (PKJI) of 2023 concerning the performance of road sections and the Directorate General of Land Transportation of 1996 concerning parking. The results of the analysis, namely the existing conditions obtained a V / C ratio of 0.43 level B, the construction period obtained a V / C ratio of 0.45 level C, the operational period with a V / C ratio of 0.45 level C, and after 5 years of operation the V / C ratio is 0.58 level C which means that the flow is stable, but the operating speed begins to be limited based on the level of service (LOS) table. Parking needs based on the building area of 2832 m2 were obtained as many as 113 srp, while based on the available parking area it can accommodate as many as 121 srp, so the parking space requirements of the Padang Eye Center Special Eye Hospital have been met. The alternative solution to this analysis is the placement of regulatory officers in front of the entrance and exit of the Khatib Sulaiman Road intersection during the construction period, regulating parking circulation within the hospital area, and making barrier gates to regulate vehicle traffic flow.

An Indoor Air Quality Analysis in Newly Constructed Apartments

This study analyzes the indoor air quality inspections of newly constructed apartments by the Busan Institute of Health and Environment for the last three years (2021-2023). Of the 41 inspections, 33 exceeded the recommended standard, and the average excess rate was 80.5%. When classified by measuring households, 148 out of 332 exceeded the standards, whereas, the average excess rate by household was 44.6%. Benzene was exceeded in two out of the 332 households; therefore, the excess rate was only 0.6%. However, the relative standard deviation was the largest, showing as the highest item in the scattergram. The average concentration of toluene was 1,006.0 µg/m3, which was 100.6% of the recommended standard and was exceeded by 138 households; hence, the excess rate was 41.6%. All six items(Formaldehyde, Benzene, Toluene, Ethylbenzene, Xylene, Styrene) reached the highest concentration in spring or summer when the indoor temperature increased. The average concentration of toluene in July was 1,922 µg/m3, close to 1.9 times the recommended standard; toluene is considered an item of particular concern in summer. In the correlation analysis with indoor temperature, the formaldehyde coefficient of determination was 0.4855, regarded as somewhat significant; it was difficult to confirm the correlation with temperature for the other five items, including benzene. In correlation with relative humidity, formaldehyde showed the same tendency as indoor temperature, with a coefficient of determination of 0.4404, which is believed to be due to its small molecular weight and high solubility. Due to the chimney effect, high concentrations of volatile organic compounds could be confirmed at low atmospheric pressure in the upper floors, and this trend was most evident for formaldehyde. For most items(Formaldehyde, Benzene, Toluene, Ethylbenzene, Styrene), a negative correlation was confirmed in which the emission amount decreased as the number of households increased. For large apartment houses, it is believed that it is due to sufficient bake-out and flush-out according to the adequate construction period and the use of eco-friendly and high-end building materials. Small apartments are classified together with many small houses. The emission concentration of small houses is relatively high because the proportion of the surface area where pollutants are released is high.