Heavy metals in urban dust inside and outside homes during the Covid-19 pandemic in Mérida, Yucatán, México

The COVID-19 pandemic caused a decrease in outdoor activities but an increase in indoor ones. Does this situation cause changes in pollution patterns? The objective of this study was to compare heavy metal contamination in indoor and outdoor dust in homes in the city of Mérida, Yucatán. Dust was collected in 51 homes on the weekends of May 2020 during the COVID-19 pandemic when there was a lockdown. Three hundred ninety-six samples were obtained (203 indoors and 193 outdoors on the sidewalk). Heavy metal concentrations were measured in triplicate with a portable XRF Genius 7000 spectrometer from Skyray Instruments. The contamination factor was calculated using a local (decile 1) and a global background value. To identify whether there were differences in indoor and outdoor heavy metal concentrations, mixed linear models were used, and the statistical inference was made using hypothesis tests. The risk to human health was evaluated using the USEPA methodology. Using decile one as background, the contamination factor's median showed moderate contamination for Lead (Pb), Manganese (Mn), Iron (Fe), Calcium (Ca), Strontium (Sr), and Yttrium (Y) indoors and outdoors. Using world concentrations as background, higher Ca, Sr, and Y concentrations were found outdoors, while Copper (Cu), Pb, Titanium (Ti), and Zinc (Zn) concentrations were higher indoors. Pb represented a risk of developing health problems for children inside homes. These studies help design public policies to reduce urban emissions and pollution, implement dust and risk management programs, and conduct citizen cleaning campaigns.


Introduction
In the last two decades, heavy metal pollution has been reported in cities worldwide.(Apeagyei et al. 2011;Chen et al. 2016;Cheng et al. 2018;Aguilera et al. 2021;2022).In urban dust, fine particles mixed with high concentrations of heavy metals abound (Hassan 2012;Aguilera et al. 2022).Therefore, urban dust can be an important route of exposure to heavy metals for the inhabitants of a city (Hassan 2012;Hogervorst et al. 2007;Yoshinaga et al. 2014).
Due to the toxicity, persistence, and bioaccumulation of heavy metals, prolonged exposure to contaminated dust can affect human health.Lead (Pb), for example, has a half-life of four years in the human body and up to 10 years in bones (Needleman 2009).Zinc (Zn) and copper (Cu) are essential elements for humans (Nriagu 1988).Pb has been identified as a probably carcinogenic metal for humans (Aminiyan et al. 2018;Tchounwou et al. 2012).
Contaminated urban dust is present in outdoor and indoor environments (Delgado et al. 2018, Delgado et al. 2019;Cortes et al. 2017;Pan et al. 2017;Santoyo-Martínez et al. 2022;Aguilar et al. 2021).Previous studies indicate two main ways of introducing dust into homes: dust that adheres to footwear and is carried into the room, and 2) particles suspended outside, which are dispersed indoors (Hunt et al. 2006;Thatcher and Layton 1995).Some studies indicate that adults and children spend approximately 88% and 75% of the day indoors, respectively (USEPA 2011).
There are several investigations on the evaluation of the health risk due to heavy metals in indoor and outdoor dust.In Istanbul, Turkey, there are records of levels that exceed the permissible content of Cr by ingestion, followed by dermal contact and inhalation in children and adults in closed and open spaces (Kurt-Karakus 2012).In Lahore and Sargodha, Pakistan, high concentrations of Cd are considered a risk for children, and Pb for children and adults, are detected inside homes (Mohmand et al. 2015).In Japan, high concentrations of Cd, Cu, Mo, Pb, Sb, Sn, and Zn in urban dust represent an increased risk for children by inhalation in schools and offices (Yoshinaga et al. 2014).In Ogun State, Nigeria, Cr represents a high risk by intake, in children and adults, inside homes (Olujimi et al. 2015).In Rouen, France high levels of Hg, Cr, and Pb inside a natural history museum were recorded (Marcotte et al. 2015).There are few studies on the differences between the concentrations of heavy metals in urban dust inside and outside houses; it is possible that each city is a case study and that the intensity of human activities causes these differences.The analysis of heavy metals in urban dust is instrumental in avoiding health risks to people.
During the COVID-19 pandemic (May and June 2020), the Mexican government and the state government implemented restrictive measures on outdoor activities (home office, online classes, traveling outside the home only in cases of urgency or essential work), so people spent a lot of time indoors.Therefore, there was greater exposure to indoor dust.It is essential to pay attention to the accumulation of heavy metals and the potential environmental and human health risks outdoors and indoors.
The objective of this study was to compare the contamination by heavy metals in the interior and exterior dust of houses in the city of Mérida and to evaluate the possible risks to human health using the USEPA methodology.It should be noted that this study was conducted during the COVID-19 pandemic.

Study area and sampling design
Mérida is located 10 m above sea level, with an urban area of 858 km 2 .The population consists of 1,100,000 inhabitants, around 480 thousand vehicles circulate, and more than 400 small and medium industries contribute to environmental pollution.The type of climate is Aw 0 (warm sub-humid with rains during the summer).Its temperature varies from 24 to 26°C.The hottest season is from April to August, it should be noted that it reaches a temperature of 40° C. The rainy season starts in June and ends in October, and the dry season is from February to May.Its rainfall ranges from 800-1000 mm (Delgado et al. 2017).The dominant soil group is Leptosol (INEGI 2018;Bautista et al. 2015).
The sampling of urban dust outside and inside the houses was carried out through citizen participation in 51 houses (Fig. 1).The samples were collected on the weekends of May and the first days of June 2020, during the dry season.Three hundred ninety-six samples were obtained (203 indoor and 193 outdoor).The samples analyzed are part of a national project titled "Heavy metal pollution in urban dust indoor and outdoor in 10 cities before, during and after COVID-19: sources and routes of entry into the human body".The dust samples were taken by citizens interested in the project.The sampling of urban dust was carried out by sweeping 1 m 2 of sidewalk surface and the total indoor surface.Before collecting urban dust, accessory components such as rocks, branches, and leaves were manually removed.The urban dust was distributed in four mounds with a brush and deposited in labeled and georeferenced polyethylene bags.
The following data from indoor the houses were recorded: 1) the number of people living in the house, 2) construction material, 3) wall material, 3) the existence of a garden or potted plant, 4) activities carried out commercial or agricultural, 5) presence of pets, 6) frequency of sweeping, 7) type of stove, 8) change of footwear when entering the home, 9) presence of sick inhabitants, and 10) sweeping area.
For two weeks, the samples were kept in a cool and dry place away from direct sunlight and air to prevent the oxidation of the minerals present (Morales et al. 2020).

Sampling preparation and measurements
Sample preparation for XRF measurements was performed using the general guidelines of the Genius 7000XRF Portable Spectrometer User Manual from Skyray Instruments.The dust samples from indoors and outdoors were sieved using a 60 mesh, and a 30 mesh was used when the amount of sample was insufficient.Three grams of dry urban dust powder sample was placed in a Teflon cup with a 3.6 μm thick Mylar (polyester) film bottom window.The concentrations of nine different elements (Ca, Cu, Fe, Mn, Sr, Pb, Ti, Y, and Zn) were determined with the following conditions: 50 kV X-ray tube with a drift detector large-area beryllium window (DDS) in the facilities of the University Laboratory of Environmental Geophysics (LUGA) of the National Autonomous University of Mexico (UNAM).For each sample, three repetitions were performed (with an integration time τ of 60 s each).Therefore, according to the procedure described above, the concentration results of each sample did not take more than five minutes.
The internal reference material IGLs-1 (a lateritic soil sample from a set of eight geochemical reference materials (Lozano and Bernal 2005) was obtained for control purposes and was systematically measured after 25 measurements of urban dust samples.Once the elemental concentrations were determined, their precision and accuracy were estimated.
In addition, representative samples of the interior and exterior of urban dust were selected for X-Ray Diffraction (XRD) analysis using a Bruker D-8 Advance diffractometer, with a monochromatic Cu Kα (λ = 1.5418Å) radiation, passage time of 0.5 s, size of 0.02 degree, at 40kV and 30mA.To carry out the above, the powders were placed on a silicon sample holder coated with appropriate vacuum grease for XRD.

Analysis of contamination levels
The degree of contamination was estimated by the contamination factor (CF), defined as: where C n is the concentration of the analyzed metal in each sample (for example, Pb, Cu, etc.) and B n is the background value for the metal under study.A CF less than 1 indicates insignificant contamination, 1-3 is moderate contamination, 3-6 is considerable contamination, and greater than 6 indicates high contamination (Ihl et al. 2015).There are no established background values for indoor and outdoor dust; therefore, the first decile was used as a background value.
(1) CF = Cn∕Bn Unlike the minimum value, the first decile allows for some variation and tolerance (Aguilera et al. 2021); the median or mean should not be an appropriate background value as it is taken from the actual samples instead of a reference population which is almost impossible to find in urban areas.In addition, the global background values for soils (Kabata-Pendias 2011) were used to compare the homes' interior and exterior.
The PLI (Pollutant Load Index) was also calculated to test for contamination by the five heavy metals with the highest CF, as developed to avoid underestimating the contamination level if multiple elements with low CF were included in the calculation.A PLI value < 1 indicates that the heavy metal load is below the background, while a PLI > 1 indicates contamination (Merhr et al. 2017;Tomlinson et al. 1980).

Comparison of outdoor and indoor contamination
A mixed linear model was used to test the effect of sample type origin (inside and outside houses).The date of collection and the fraction that was measured (595 and 250 μm) were also included as fixed effects, solely to control their contribution to the variation of the model.The variance contributed by the location of the houses was counted as a random effect on the intercept since the observations taken in the same house were not independent of each other.The linear mixed model was fitted with the lmer function from the lme4 package of the R project software, version 4.1.1(2021-08-10) "Kick Things".Subsequently, it was visually proven that the assumptions of the model were fulfilled, i.e., normality and homoscedasticity of the residuals, as well as the normality of the random effects.When these assumptions were not met, a generalized linear mixed model with gamma distribution (glmer function from the lme4 package) or a linear mixed model with logarithmic transformation was used.Finally, the statistical inference was made on the effects of the type of collection, the date, and the fraction, using hypothesis tests with the Anova type II function of the lme4 package.
In addition, a one-way analysis of variance with the Kruskal-Wallis test was performed only on particles of 60 mesh, the factor was the site (indoor, outdoor), and the variables were heavy metal concentrations.

Health risk assessment by heavy metals in urban dust outdoor and indoor
The methodology developed by the Department of the Environment of the United States of the Protection Agency (USEPA) was used to estimate the health risk to the population of Mérida.The estimated daily intakes for the three main exposure routes: ingestion (EDIing), inhalation (EDIinh), and dermal contact (EDIdermal), as well as lifetime average daily dose (LADD), were calculated, the latter to estimate the carcinogenic risk (Aguilera et al. 2021).In this study, the three routes were analyzed, as shown below: (3) All exposure factors used in this study are those established for reference populations.CR is the contact rate (or absorption); CR = IngR for ingestion, CR = InhR for inhalation, and CR = SA × AF × ABS for dermal contact.The type of CR used for each carcinogenic metal depends on the route of exposure by which it can cause cancer.
Hazard indices for ingestion, inhalation, and dermal contact (HQ ing/inh/derm) were obtained by dividing the EDI by the reference dose (RfD) as shown: The Hazard Index (HI) represents the sum of the HQs for the three routes of exposure.If the HI is > 1, there is the possibility of producing a non-cancer risk in the health of the population; while if it is < 1, the opposite would be expected (USEPA 2001).
The estimated daily intake (EDI) in mg/kg per day by ingestion (EDIing), inhalation (EDIinh), and dermal contact were divided by their corresponding reference doses (RfD) to obtain the risk quotient for each route of exposure (HQing / inh/derm), and finally, the non-cancer hazard index (HI) was found by adding all those HQing/inh/derm.A non-cancer hazard index (HI) greater than 1 indicates that possible adverse (7) HQing∕inh∕derm = IDEing∕inh∕derm RfD effects may occur on human health.The same exposure factors, reference doses, and cancer slope factors reported in Aguilera et al. (2021) were used indoors and outdoors.For comparative purposes, the same exposure time was considered indoors and outdoors.However, as people spent almost all day indoors, the estimated health risk outdoors was overestimated.On the other hand, the model does not consider exposure hours per day, only exposure days per year, therefore it is difficult to determine the total exposure time in each site.
The LADD was multiplied by the slope factors (Aguilera et al. 2021) to obtain the incremental lifetime cancer risk.The accepted or tolerable cancer risk is in the range of 1E-06 to 1E-04 (USEPA 2001).These values indicate that one additional case in a population of 1,000,000 and 10,000 people is acceptable (Lu et al. 2014).The analyzes were performed with the R Project software, version 3.52.

Results
Outdoors, the highest coefficients of variation corresponded to Pb and Cu; indoors they corresponded to Cu, Pb, and Zn.Those features indicate that the concentrations of these elements have been enriched by the anthropic activities of the city of Mérida (Aguilera et al. 2021), such as the vehicle fleet, wear of auto parts, brakes, and tires, as well as the industrial activity of the city (Jiang et al. 2018).
Table 1 shows a descriptive statistical summary of the analysis of the registered concentrations of each element (Ca, Cu, Fe, Mn, Sr, Pb, Ti, Y, and Zn), both for the exterior and interior of the homes, as well as the global background values for soils and the first decile of the frequency distribution for each element.Outdoors, the background values of the first decile for Pb, Cu, and Zn exceeded the world background values for soils.Indoors, the first decile background values of Pb and Zn exceeded those for soils worldwide.This result indicates that these elements from the dust inside and outside homes come mainly from anthropogenic sources.In studies of environmental contamination, it is common to find that the pollutant has values above the global threshold mean for soils, even values considered atypical, with different values between the median and the mean, as well as high values of the standard deviations.By contrast, heavy metals from natural sources tend to fall within the Gaussian distribution, with nearly equal values between the median and mean, as well as low standard deviation values.

Contamination level
Using global background values for soils, the outdoor concentrations of Pb, Mn, Fe, Ca, Sr, and Y registered moderate levels of contamination (CF > 1).In contrast, Cu, Zn, and Ti registered considerable levels of contamination (CF > 3).Indoors, Pb, Mn, Fe, Ti, Ca, Sr, and Y registered moderate levels of contamination (CF > 1), while Cu and Zn recorded considerable contamination levels (CF > 3).Therefore, urban dust from inside and outside homes is contaminated with heavy metals (Fig. 2).
When the background value for soils was established worldwide, a greater variation was observed in the levels of contamination both outdoor and indoor.The element with the highest CF was Zn in the considerable and high contamination category, followed by Sr, Cu, and Y in the moderate contamination category.The elements Mn, Ti, and Pb were located in the category of insignificant contamination for outdoors.For indoor, the element with the highest CF was Zn, located in the category of high contamination, followed by Cu in the category of considerable contamination.Pb, Sr, and Y were located in the moderate contamination level.The elements Mn and Ti were located in the insignificant category, indoors (Fig. 2).The dust from Merida's indoor and outdoor is contaminated by heavy metals, regardless of the background value used in this study.A PLI greater than 1 was obtained, indoors and outdoors, using the first decile as the background value.For the soil global background value, the elements Cu, Pb, Zn, Fe, Ca, Sr, and Y presented a PLI greater than 1 indoors and outdoors, and the elements Mn and Ti had a PLI lower than 1.It should be noted that it is difficult to establish the background values (natural) of the elements when it comes to dust, especially when it is located in urban sites full of human activities.

Comparison of indoor and outdoor contamination
The fixed effects results suggested that the type of collection (indoor and outdoor) and the size of the analyzed fraction had relevant and significant effects (Table 2).Higher concentrations of Ca, Sr, and Y were found outside the houses; while the concentrations of Cu, Pb, Ti, and Zn were higher inside the homes.In the case of the type of fraction analyzed, higher  concentrations of Fe, Sr, Ti, Y, and Zn were found in the 595 μm fraction, and this makes sense since this incorporates the 250 μm fraction.This factor was only considered in the models to assess the variation they could affect, but there was no research question behind it.The random-effects component of the models suggested that the effect of the house variable (location) was quite important since the standard deviation associated with this factor was very similar to the standard deviation associated with the residuals.In other words, the variance associated with the house was not negligible (Fig. 2).
When we use only the 60-mesh fraction, we find the same result: higher concentrations of Ca, Sr, and Y outdoors and higher concentrations of Cu, Pb, and Zn indoors (Fig. 3).Other elements did not show significant differences between their indoor and outdoor concentrations.
The spatial analysis showed that the site with the highest CF recorded for Pb and Zn outdoors, as well as for Pb indoors, was located southeast of the city (site 36, Fig. 4); Close to this site it was possible to locate a metal buying and selling establishment and a scrap yard (Fig. 1).The highest CF for Zn and Ti indoors were located at site 15, south of the city, next to the international airport.
The highest CF of Cu, both outdoors and indoors, corresponded to site 48, located in the city's center with proximity to a shopping center and an education center, which could be related to high vehicular traffic.The highest CF of Mn and Fe abroad matches with site 42, located north of the city, close to an automotive workshop that includes bodywork and painting services.The highest CF of Mn and Fe indoors corresponds to site 17 located to the northeast.
On the other hand, the highest PLI value outdoors corresponded to site 36, and indoors to site 42.Both locations also presented high CF values for various elements, so it is advisable to consider some mitigation measures and alert their inhabitants.
In one of the most contaminated sites (site 48), the inhabitants reported a monthly sweep frequency, the most prolonged period recorded for all the sites.However, when comparing the other parameters of the houses, no other relevant differences were identified that could explain the different concentrations of heavy metals reported in this study.

Human health risk
The average HI of Pb, Cu, Fe, Mn, and Zn for the interior and exterior of homes, both for children and adults, registered a value of less than 1, indicating no health risk.There is a greater risk for children than adults, especially indoors, in Merida (Fig. 5).Although the maximum hazard index value (HI = 3.05) was found outside, it was an outlier.For comparative purposes, the same exposure time was assumed indoors and outdoors; however, in practice, people spent most of the time inside their homes.Therefore, the actual outdoors HI must be smaller than we presented.
It should be noted that lead is considered one of the longest-lasting heavy metals in cities (more than 20 years), devastating for children (Martins 2021).It is necessary to carry out future studies to corroborate the risk and alert the population.
Another essential aspect to consider is that we use the dust fraction of 250 microns in the risk analysis; on the one hand, the particles are not of respirable size, weak particles that could achieve it due to weathering.On the other hand, no heavy metal concentrations can be considered safe for humans, neither for ingestion nor inhalation.Therefore, the values of the risk indexes reported here are only a very general indicator that can be used to take precautionary measures.

X-Ray Diffraction Analysis (XRD)
The presence of calcite (CaCO 3 ) was the main mineral that predominates both in the exterior and interior of homes in the city of Merida.Figure 6 shows an example of a typical analysis of a sample from outdoors and indoors.It should be noted that calcium is an element found in abundance in nature due to Yucatan's type of karstic soil.Therefore, it is not considered a risk to human health.
Inside the houses, there are traces of minerals such as gypsum and aluminosilicates such as nacrite and talc that are used in the construction of houses and daily cleaning; respectively, they were not found outside, and traces of silica and hematite were identified outdoors.

Discussion
In this study, contamination was analyzed using a local and a global background value to compare the contamination values of the analyzed metals.The global background value serves as a reference at a general level with other urban areas.On the other hand, the local background value provides us with in situ information on the study area (Aguilera et al. 2022).
Regardless of the background value, the Pollutant Load Index (PLI) indicated moderate contamination (> 50% of the data) by heavy metals in Merida dust, both outside and inside the houses.It is recommended to use both If low concentrations were found in the dust, the local background value (decile 1) would be low, and the contamination factors would be high.It will result in greater contamination than the one found with the global background value (established for soils worldwide).This result was the case for Merida; with the local background value, it seems that there is greater contamination, except for Zn, which had more significant contamination with the global background value.
Merida is a city with low concentrations of heavy metals; naturally, local background values are usually lower than those reported for soils worldwide (Aguilera et al. 2021(Aguilera et al. , 2022)).Therefore, the contamination with the local background value tends to be higher than the one with the global value.
At first glance, no differences between indoor and outdoor heavy metal contamination were observed with the local background value; since this background value was taken from the concentrations' distribution of frequencies, which causes the contamination to remain around the category of moderate contamination.However, with the general background value (soil worldwide), it was possible to observe differences between the concentrations indoor and outdoor.
According to the mixed linear models, significant differences were found in the concentrations of metals indoor and outdoor.The concentrations of Ca, Sr, and Y were higher outdoors; while the concentrations of Cu, Pb, and Zn were higher indoors.Ca, Sr, and Y are found in the minerals of the sedimentary rocks and found in the soils of the city (Bautista et al. 2003(Bautista et al. , 2011) ) and dust (Aguilera et al. 2021).Cu, Pb, and Zn are widely reported elements of anthropogenic origin in many materials.
We expected heavy metal concentrations to be higher outdoors since urban dust is a source of indoor dust.However, the results suggest that other emission sources inside the houses contribute more than the outside ones or that the concentrations inside the houses accumulate due to less dispersion.Higher heavy metal concentrations have also been identified indoors in Neyshaburcity, Iran; moreover, the authors observed higher cytotoxicity indoors than outdoors (Naimabadi et al. 2022).Cigarette smoking indoors (Latif et al. 2009), poor ventilation (Praveena et al. 2015), energy use, and cooling Fig. 5 Box plots for noncarcinogenic risk indices (HI), indoors and outdoors, for children (upper image) and adults (lower image).The dotted line indicates the threshold from which health problems can be triggered (Rasmussen et al. 2001;Lin et al. 2017), could explain an increase in the concentrations of heavy metals such as Cu, Pb, and Zn in the dust inside homes.During May, the average maximum temperature in Mérida is 36º C (96.8º F), which is why cooling equipment is used in the houses in the north and center of the city, resulting in poor ventilation and possible dust accumulation.Another possible explanation is that outdoor concentrations decreased as a result of national and state mobility reduction programs (closure of schools, home offices, vehicle traffic restrictions, etc.).Indeed, it has been reported that COVID-19 lockdowns caused a reduction in air pollution (Venter et al. 2020).A comprehensive study in 87 capital, industrial and polluted cities of the world also supported this reduction in air pollution (Sarmadi et al. 2021).
Pb is a common metal in the home, such as lead paint, ceramic glazes, and batteries (Yadav et al. 2019;Zhao et al. 2020).Similar results were obtained in another heavy metal contamination study by Li et al. (2001).They attributed the high concentrations of Pb indoors to the use of Pb in common objects at home.Al-Madanat et al. (2017) also found higher concentrations of Cu indoors.Still, those of Pb and Ti were higher outdoors after reviewing indoor and outdoor dust samples from residential and commercial sites in Al-Karak City, Jordan.They concluded that the concentrations of V, Ti, Mn, Pb, and As came from vehicle emissions, except for Ni, Cr, and Cu, which were found in higher concentrations in indoor dust samples.
In Merida, we found a greater health risk (HI > 1) indoors, and ingestion was the main route of exposure.Pb represented a risk, especially in children indoors.Pb is one of the most important heavy metals for public health in Mexico (Hu et al. 2011;Caravanos et al. 2014;Aguilera et al. 2022).In addition, it is crucial to consider that the population spends more than 75% of the time indoors (USEPA 2011).Especially at the beginning of the coronavirus disease, when the population was told to stay home as long as possible.Pb is destructive to the nervous, kidney, circulatory, and reproductive systems, especially for children (Needleman 2009).For this reason, it is essential to remove Pb from home products, especially paints and enamels.
In China, ingestion was also the most critical route for exposure of Pb, Zn, Cu, and Ni from household dust, and dermal contact was identified as a major route for Cr and Cd in residents' exposure.There were minor non-cancer and carcinogenic risks from heavy metals in urban dust (Li et al. 2004;Wei et al. 2010;Huang et al. 2014;Cheng et al. 2018).
Pb represented a risk to the health of children indoors.Therefore, it is recommended to review the houses to obtain more relevant information about the age of construction, type, and color of paint, and material of the pipes, among others.Chronic exposure to Pb triggers serious health problems, with children being the ones at a greater risk since they are in direct contact with the different surfaces of the house.These findings will help launch public policies to reduce the pollutant load of heavy metals indoors and outdoors.Government actions to reduce emissions need to be carried out, as well as urban dust management programs, citizen cleanup campaigns, and potential health risk management programs due to contact with urban dust and heavy metals.
Calcite, quartz, gypsum, aragonite, and talc were the main dust minerals typical of sedimentary rocks (Aguilera et al. 2021).For this reason, we find Ca and Sr in urban dust.Sr concentrations were well above the global average concentration.
Since there is a risk of dust for the city of Merida population, it is possible to consider placing more emphasis on indoor and outdoor cleaning.It should be noted that there is no previous history of studies of urban dust with heavy metals inside houses in Merida.The results were surprising and, at the same time, alarming due to the considerable Cu and Zn outdoors and moderate concentrations of Pb, Mn, Fe, Ti, Ca, Sr, and Y indoors.
Daily contact with these metals represents a long exposure time that increases the health risk.It should be noted that with the coronavirus disease, families mainly remained inside their homes.These findings indicate us to research possible sources of human contamination and the conditions of the houses, to carry out constant monitoring of urban areas and the people who live in them to avoid risks to the population's health.
This research reveals relevant information to alert citizens and governments to decrease exposure to heavy metals.Studying heavy metals in urban dust can help generate and apply policies to reduce human emissions of the various heavy metals studied, particularly those known to be more harmful to children and adults.With this, it is presented to alert multiple populations of risks to human health.

Conclusions
Urban dust from indoors and outdoors in Merida, Yucatan, Mexico, recorded the presence of Pb, Mn, Fe, Ca, Sr, Y, Cu, Ti, and Zn.
Outdoors Mn, Fe, Ti, Ca, Sr, Y, and Zn presented moderate contamination, while Cu and Pb registered considerable contamination.
Indoor Pb, Mn, Fe, Ti, Ca, Sr, and Y registered a moderate level of contamination; however, Cu and Zn registered a considerable contamination level.
The concentrations of Cu, Pb, and Zn were higher indoors, with significant differences.
More research on urban dust is necessary, mainly for the identification of sources of heavy metals inside houses, as well as the study of the respirable fractions of urban dust.Pb represented a risk to the health of children in closed spaces.Therefore, it is recommended to review the houses to obtain more relevant information about the age of the construction, the type and color of paint, and the material of the pipes, among others.In this way, a government plan could be implemented to identify the houses with greater exposure to lead and design an action plan for its reduction.

Fig. 2
Fig. 2 Box plots for the contamination factors of heavy metals inside and outside the houses, using the first decile as background value (upper image) and the soil worldwide background (lower image)

Fig. 4
Fig. 4 The geographical location of contamination factors by elements indoor and outdoor

Table 1
Statistical summary of heavy metal concentrations in mg/kg and background values outside and inside the houses D1 decile 1, Min minimum, Max maximum, SD standard deviation, CV coefficient of variation, Kabata-

Table 2
Model selection and results for each element with significant outputs