1 Introduction
formaldehyde is one of the probable carcinogens.Based on cohort mortality studies of workers exposed to formaldehyde with an increased incidence of nasopharyngeal Cancer,the International Agency for Research on cancer classified formaldehyde as a Group 1 carcinogen for humans in 2004
[1,2]。 Further studies indicate that long-term exposure to formaldehyde with relatively high concentrations causes nasopharyngeal cancer,leukemia,and many other types of cancers
[3,4]。 Moreover,the carcinogenic effect of formaldehyde increases with the coexistence of other indoor pollutants such as BTEX,halogenated hydrocarbons,polycyclic aromatic hydrocarbons,and carbonyl compounds
[5]。 Occupational exposure to formaldehyde has adverse effects on semen quality,particularly on sperm motion parameters
[6]。 Exposure to formaldehyde also has adverse reproductive and developmental effects,leading to miscarriage,congenital monstrosity infants,low birthweight infants,and premature infants
[7,8]。 Moreover,the damage caused by formaldehyde for infants and children cannot be recovered
[9]。 Hence,indoor formaldehyde pollution causes widespread fears in China,especially after the so-called“formaldehyde room case,”in which the formaldehyde concentration of a long-term rental apartment exceeded the national standard limit,resulting in the death of tenant
[10,11]。
Indoor formaldehyde pollution is becoming more significant because of economic development and more people decorating their homes.Almost all furnishing materials such as artificial boards,paints,resins,and furniture release formaldehyde with the duration of several years to decades,making the abatement of indoor formaldehyde a long-term task.China published the national standard“Indoor Air Quality Standard,”GB/T18883-2002 in 2002,which adopts the World Health Organization-recommended upper limit concentration of 0.1 mg/m
3(approximately 0.08 ppm)for indoor formaldehyde.The enactment of the standard promotes the control of indoor formaldehyde pollution and helps the improvement of indoor air quality;nevertheless,the monitored indoor formaldehyde concentrations seem not satisfying
[1][12]。 There is more work needed to abate indoor formaldehyde。
Note that the accumulated data on indoor formaldehyde concentrations is undoubtedly the basis for analyzing the influencing factors and for choosing appropriate abatement strategies for different types of residences.Therefore,this paper collected indoor formaldehyde concentrations of a wide series of decorated rooms from various cities in China from published literature,and the indoor formaldehyde pollution status and the major influencing factors were examined。
2 Methods and dataset
the dataset was obtained from The literature that reported indoor formaldehyde concentrations in decorated rooms in Chinese cities。
Since the literature reporting indoor formaldehyde concentrations was mostly published in Chinese,these references were mainly obtained by searching in cnki(China National Knowledge Infrastructure,www.CNKI.net)using the keywords“formaldehyde,”“indoor,”and the name of Chinese provinces or cities.the literature search was performed province by province with the publishing time from 2003 to 2019.For example,literature about Beijing was searched using the keywords“indoor and formaldehyde and Beijing”and period“2003—2019,”whereas literature about Heilongjiang was searched using the keywords“indoor and formaldehyde and Heilongjiang”and period“2003—2019,”and so on.After the province-based search,literature about Qinghai,Guizhou,Yunnan,Hebei,Jiangxi,Zhejiang,Fujian,Taiwan,Hong Kong,and Macao was not found.Then,a city-based search was conducted with the same process except that the names of the capital cities of these provinces were used.Unfortunately,only the literature about Guizhou(Guiyang),Hebei(Shijiazhuang),and Jiangxi(Nanchang)was found in this process.Finally,a review-based search followed,and the literature about the rest provinces(except Taiwan,Hong Kong,and Macao)was added from the reference lists of three related reviews
[12⇓~14]。 After all the above searching processes were finished,295 literature were collected.Since only the formaldehyde concentrations in decorated houses in urban China were considered in this paper,the literature on public places and countryside was excluded,and 221 were left。
to cover more provinces and balance the sample size among provinces as possible,the literature was chosen on the basis of the following criteria:i)Choose data from as many provinces as possible.ii)Select data in different test years within the same province/city.To obtain more information on indoor formaldehyde,the references were further screened as follows:iii)Select those pieces of literature containing average concentrations,lowest concentrations,highest concentrations,and over-standard rates in priority.iv)Select works of literature with both average concentrations and over-standard rates before those reporting only average concentrations or over-standard rates.v)Select pieces of literature with only the highest and lowest concentrations at last.According To the above principles,67 pieces of literature were finally selected,from which a dataset was extracted.the dataset contains 10 394 dwellings with 27 384 rooms in 62 cities in 31 provinces in China.Fig.1 illustrates the flowchart of the bibliographic search。
Since the formaldehyde concentration may vary among rooms even in the same residence and the influence of room type will be analyzed here,we chose the number of measured rooms as the sample size,and the“sample size”refers to the number of rooms in the following text.Table 1 summarizes the details of the dataset,including provinces,cities,locations(the south or the north),test years,test months/seasons,sampling temperature,sampling methods,sample sizes,room types,time after decoration,average Concentrations,standard deviations,the lowest and highest concentrations,and over-standard rates.concentrations of offices,schools,public places(e.g.,markets,entertainment venues,hotels,hospitals,etc.),and factories were not considered in this review.Readers could refer to other papers for such information
[12]。
the weighted average of each value(concentration or over-standard rate)was used for data analysis.When considering the over-standard rates of different types of rooms,the over-standard rates belonging to the same type of room were weighted and averaged,and the results of all types of rooms were compared.the weight was calculated as the ratio of each over-standard rate's sample size to the sum of the sample sizes for the same room type.the details are presented in the captions of figures。
3 About the dataset
In indoor formaldehyde analysis,both derivatization and instrumental methods(or the sensors)are commonly applied
1.In the 67 selected studies,32 used the derivatization method,and 26 used the sensors to determine formaldehyde concentrations(with nine studies unspecified).Fig.2 shows the distribution of rooms with different methods.The derivatization methods were more frequently adopted(Fig.2a)because of the wide recommendation by Chinese National Standards(Table 2).The sensors,however,could provide the same accuracy as derivatization methods with regular and careful calibrations;therefore,they are also recommended by Chinese National Standard GB/T 18204.2-2014 and Environmental Standard HJ/T 167-2004.Liu et al.evaluated the derivatization method(MBTH)and instrumental method(PPM sensor)in various concentrations(0.01~1.38 mg/m
3)at temperatures ranging from 5℃to 35℃and concluded that no significant differences between these two methods.Since Interscan sensors are more commonly used than others(Fig.2b),they were also compared with the MBTH method,and the results were consistent with each other.Therefore,the data from derivatization methods and sensors were treated equally in this paper without any correction
[82][83,84]。
It is worth mentioning that the most frequently used method in derivatization methods is MBTH(Fig.2c);however,this method is nonselective.other aldehydes undergo an analogous reaction as formaldehyde even in a lower yield,so the MBTH method might overestimate the formaldehyde concentration in the presence of Other volatile aliphatic aldehydes
[85]。 Giesen et al.concluded that measured values from MBTH method showed almost no correlation with values from the acac and DNPH methods at formaldehyde concentrations within 0.1 mg/m
3;however,Huang et al.stated that results from MBTH,chromotropic acid,and AHMT methods showed consistency even chromotropic acid method was unsuitable for low concentrations
[85][86]。 Nonetheless,fortunately,most of the measurements(40 studies,see Table 1)follow the standards that are listed in Table 2(with 26 studies unspecified,one study followed the author's previous method).deviations definitely exist although the measurements followed the national standards;large Deviations among the data are also not expected.Therefore,we treated all the data as reliable results and analyzed them accordingly。
4 Indoor formaldehyde pollution status
The distributions of average concentration,highest concentration,and over-standard rate of indoor formaldehyde were generated,as shown in Fig.3,using the weighted averages of indoor formaldehyde parameters for a province as listed in Table 1.The average concentration covers 31 provinces(the Taiwan,Hong Kong,and Macao data were not available),the highest concentration covers 29 provinces,and the over-standard rate covers 31 provinces.The average concentrations of the provinces except five exceeded the maximum limit of 0.1 mg/m3recommended by the national standard GB/T 18883-2002,and the highest one even exceeded almost 5 times the limit.All the highest concentrations exceeded the limit except in one province,and the highest one was even as high as 25 times the limit.Meanwhile,except for one province,all the over-standard rates were above 10%,and more than half of them were higher than 50%.The results in Fig.3 show that formaldehyde pollution of indoor air is a severe issue across China 。
It is worth noting that the distributions in Fig.3 might deviate from the real situation because the sample size and type(room type,time after decoration,test year and test season/month,etc.)varied largely among provinces(Table 1).Moreover,indoor formaldehyde concentrations changed with year and season;the merged results might deviate from the current condition.Nevertheless,distributions in Fig.3 intuitively reflect the fact that indoor formaldehyde pollution is very serious in urban China.More effective measures are needed to abate indoor formaldehyde to improve indoor air quality。
5 Influencing factors
5.1 Change patterns over the years
Fig.4 shows the changing patterns of indoor formaldehyde concentrations and over-standard rates over the years using the data with specific test years(2003-2018)in Table 1.The average concentrations in all test years exceeded 0.1 mg/m
3,the maximum limit concentration allowed by the national standard GB/T 18883-2002(Fig.4a).The Chinese National Forestry and Grassland Administration reported that artificial boards,which are among the most important decorating materials,increased rapidly from 2003 to 2016
[87,88]。 65%of the artificial board was consumed by furniture manufacture and 7%by wood floor production,both of which were directly related to the interior decoration.Thus,increasing decoration load with years could be expected.Since the decorative materials are the major sources of indoor formaldehyde,indoor formaldehyde concentrations would also increase with years.However,a decreasing trend was found for the average concentrations from 2003 to 2009 and reached near the limit in 2009—2013(Fig.4a).One possible reason is the restrictions on Chinese National Standards.A maximum concentration of 0.1 mg/m
3is stipulated in the national standard GB/T 18883-2002
[88][64,66,78][14]。 Moreover,the national standard GB 18580-2001“Indoor decorating and refurbishing materials—Limit of formaldehyde emission of wood-based panels and finishing products”stipulates that the amounts of formaldehyde released from artificial boards(E1)must be limited within 9 mg/100 g(for perforator test),1.5 mg/L(for desiccator test)or 0.12 mg/m
3(for chamber test).These standards undoubtedly help control the emission of formaldehyde from decoration materials and reduce the indoor formaldehyde concentration.Improvement of social-environmental awareness might be another reason 。
An increasing trend was observed from 2013 and exceeded twice of the limit in 2014,2016,2017,and 2018(Fig.4a).A similar change pattern was also observed by Huang et al.
[14]。 the authors analyzed the indoor formaldehyde concentrations in urban China from 2002 to 2015 and found the concentrations decreased from 2002 to 2009;this trend reversed during 2009—2012 and then decreased slightly in 2012—2015。
the increasing trend from 2013 was probably because of the further increased decoration load.Considering the difference in economic development levels,the increase in decoration load might be different between big and medium-small cities,so the change patterns of big and medium-small cities were compared,as shown in Fig.5a.Generally,indoor formaldehyde for both big and medium-small cities decreased from 2003 to 2012 with the concentrations of medium-small cities slightly higher than those of big cities.however,from 2014,the concentrations in big cities increased significantly and became much higher than in medium-small cities,only showing a minor increasing trend.Since the East Area in China(East China)is economically more developed than Other areas in China,we further compared the change patterns between East China and the other areas.Similarly,both the concentrations of the East and the concentrations of the other areas decreased in 2003—2013 with the latter being higher than the former,but the concentrations of the East increased significantly from 2014 and became much higher than those of the other areas(Fig.5b).the results of Fig.5 indicate that the increase in concentrations from 2013 was more from more developed areas(big cities and East areas),but the underlying reasons remain unknown.more decoration load in more developed areas might be a reason,but we do not think such a sharp increase could be fully explained by decoration load alone.other factors such as higher indoor temperature and less air change rate may also count(vide infra);However,the lack of related information made a deep discussion impossible.Furthermore,the results were based on the“reported data,”so unbalanced reports among different areas,which cannot be excluded,might also affect the conclusion.Nonetheless,the regional discrepancy in change patterns is clear,and more work is deserved on this interesting topic。
A decrease in air change rate might be another major contributor to the observed increasing trend in concentrations since 2013.Efforts to develop more energy-efficient buildings resulted in reduced air change rates with years across the world
[89]。 Inadequate air change rates result in the accumulation of pollutants and thus higher indoor formaldehyde concentrations
[90]。 Regrettably,studies on air change rates in Chinese homes began recently,and the information is scarce,a clear correlation cannot be established here
[89,91,92]。
The highest concentrations largely exceeded the national standard limit(Fig.4a);the highest was 1.78 mg/m3,which was more than 17 times the limit.Generally,the highest concentrations decreased from 2003 to 2018,which was different from the trend of average concentrations(Fig.4a).Although the average concentrations increased from 2013 to 2018,the highest concentrations still decreased in the same period.One possible explanation was Chinese National Standard GB 50325-2010“Code for indoor environmental pollution control of civil building engineering”and GB 18580,which imposed environmental regulations on building engineering and decorating materials.Another reason could be that with increased purchasing power,people could afford more environmentally friendly decorative materials 。
All the lowest concentrations were within the national standard limit;however,they were still much higher than the chronic concentration limit of 0.009~0.01 mg/m
3recommended by CA OEHHA or ATSDR
[13]。
Similar to the average concentrations,the over-standard rates decreased rapidly from 2003 to 2009,increasing rapidly since 2013(Fig.4b).Generally,the over-standard rates were very high and more than 50%in most years。
5.2 Influence of temperature and humidity
the data with test temperatures were extracted and plotted with the room temperature to assess the influence of room temperature on indoor formaldehyde concentrations and over-standard rates,as shown in Fig.6.Expectedly,concentrations and over-standard rates increased significantly with room temperature.This positive correlation was commonly found by researchers when measuring concentrations in different seasons or months.Huang et al.found a strong positive correlation between indoor formaldehyde concentration and temperature,whereas Chen et al.stated that the concentration increased approximately linearly with temperature.Besides the concentration,Zhou et al.found the over-standard rate also increased with temperature in the range of 17.3~31℃.Although deviations of detail existed between Fig.6 and other researchers'results,the trend that higher room temperatures lead to higher concentrations stays the same
[76][58][93]。
the positive correlation between indoor formaldehyde concentration and room temperature is mainly because The formaldehyde emission rates of furnishing materials increase with temperature
[97]。 the theoretical work by Xiong et al.showed that increasing temperature increases The emission rate
[98]。 Formaldehyde and total VOCs from the particleboard increased significantly with temperature according to environmental chamber experiments
[99]。 Liang et al.observed annual cyclical,seasonal variations in indoor formaldehyde concentrations and emission rates through a full-scale experiment of long-term formaldehyde emissions from medium-density fiberboard.the maximum concentration always occurred in summer because of the highest emission rate caused by the highest temperature,whereas in winter,the minimum concentration appeared simultaneously with the lowest temperature and the resulting lowest emission rate
[100]。
humidity also affects the indoor formaldehyde concentration by affecting the emission rate.the data with measured relative humidity were extracted and plotted,as shown in Fig.7,to reflect the effect of humidity.Although the data from different studies do not correlate,increasing trends of average concentration and over-standard rate with increasing humidity were observed in each investigation.This could be due to a higher rate of formaldehyde emission from furnishing materials with higher humidity.Indeed,bench-scale tests and a small field study of formaldehyde emissions from fiberglass and polyester filters showed that formaldehyde emissions increased markedly with increasing relative humidity
[101]。 Hydrolysis of adhesives or binders such as urea-formaldehyde resin by moisture seems the main reason,and this process would certainly be accelerated by increasing humidity
[102]。
Nevertheless,the effect of humidity should not be concluded on the measured results.China has a marked continental monsoon climate.seasons with higher humidity correspond to Seasons with higher temperatures,as shown in Fig.8,so the observed influence of humidity inevitably contains the influence of temperature or even just the reflection of the temperature influence.Furthermore,researchers commonly used relative humidity;however,relative humidity depends not only on the moisture content in the air but also on the temperature.the relative humidity may vary at different temperatures with the same moisture content.If these factors are ignored,inconsistencies in the relationship between humidity and concentration may occur.Wu et al.for example stated that the concentration was positively correlated with relative humidity,whereas Zhou et al.concluded that the concentration reached the maximum in the medium humidity range
[37][93]。 It can also explain the observed annual cyclical,seasonal variations in indoor formaldehyde concentrations and emission rates
[100]。 the authors observed that the concentrations positively correlated with the absolute humidity rather than relative humidity
[100]。 This is because the seasonal variation in relative humidity deviates from that of the temperature,especially in southern cities(Fig.8a),whereas the absolute humidity and temperature vary synchronously in an annual cycle,as shown in Fig.8b。
It seems meaningless to consider the effect of humidity without incorporating temperature.the humidity and temperature should be investigated together as many researchers did or be combined as one meteorological factor because the hydrolysis of adhesives and binders needs water as reactants and temperature(heat)to accelerate the process.Most studies in this field were based on modeling and laboratory tests;however,these models do not match measured data from the ambient environment with varying temperature,humidity,and property of materials
[103,104][69][105]。 Thus,more work is needed to elucidate the relationship between formaldehyde concentration and meteorological factors in real residence buildings。
5.3 Seasonal trends
the data in Table 1 with specific test seasons/months were plotted with the season to see the seasonal trend of indoor formaldehyde(Fig.9)。
Generally,indoor formaldehyde concentrations in different seasons were in the order of winter≈summer>autumn>spring,whereas the order of over-standard rates was summer>winter>autumn>spring.as discussed in section 5.2,higher concentrations should be expected in seasons with higher temperatures.summer and autumn temperatures are higher than in spring and winter,As shown in Fig.8,according to the monsoon climate in China;hence the concentrations in summer and autumn should be higher than in spring and winter.However,such a seasonal trend was not observed in Fig.9.One possible reason was the different seasonal variations between outdoor and indoor temperatures.the seasonal temperature that cyclically varies with the monsoon climate is the outdoor temperature,whereas the temperature that directly affects the indoor formaldehyde concentrations is the indoor temperature.When heating or cooling was widely applied indoors,the indoor temperature would differ a lot from the outdoor temperature and thus may vary differently from the monsoon climate。
China is divided into the south and the north by the Qinling Mountains-Huaihe River line with respect to building construction and heating supply policies.Heating is supplied in cold seasons in the north while not in the south,and the air change rate is required at least 1 h
-1in the south while only 0.5 h
-1in the north by Chinese building energy efficiency standard
[14]。 Such a situation caused large differences in indoor temperatures and ventilation rates in winter between the south and the north,leading to different seasonal change patterns of indoor formaldehyde concentrations
[14]。 the merged data of the south and the north may disturb the real seasonal trend of concentrations。
To avoid this disturbance,the data in Table 1 for the south and the north were plotted separately with the season,as shown in Fig.10.Indeed,different trends were observed between the south and the north.in the north,the order of concentration was winter>summer>autumn>spring,whereas in the south,It was summer>autumn>spring>winter.the over-standard rates followed the same order as the concentrations in both the north and the south.it is also worth noting that the concentrations and over-standard rate were much higher in the north than in the south。
the seasonal trends of concentrations and over-standard rates in the south correlated well with the annual temperature change,as shown in Fig.11b.higher average concentrations always appeared in seasons with higher indoor temperatures due to the higher formaldehyde emission rate。
It was a different situation in the north.in the north,the concentrations followed the same trend as the indoor temperatures in spring,summer,and autumn,however,the highest concentrations appeared in winter,the coldest season.This was mainly caused by the heating supply,indoor temperatures in winter in the north were much higher than the outdoor temperature(Fig.11a and Fig.8).Furthermore,the heaters were always close to furniture or covered by boards(e.g.,floor heating system),generating higher temperatures(even higher than in summer)in furnishing materials,which,of course,promoted the emission of formaldehyde。
humidity might also contribute to the seasonal change of indoor formaldehyde;however,the relative humidity did not change much across seasons(Fig.11c,d).Absolute humidity showed similar patterns to temperature and might contribute to the seasonal variations of formaldehyde concentrations;however,its effect can hardly be isolated from temperature。
Another factor that affects the seasonal changes of indoor formaldehyde is the ventilation rate.According to Hou et al.,the median air infiltration rate in both the south and the north was only 0.34
-1,which did not meet the Chinese building energy efficiency standard
[89][14]。 Ventilation did not rely on natural infiltration but opening windows.The air change rate is strongly influenced by occupants'window opening behavior
[89]。 in the north,except in summer,people tended to close windows most nights,especially in winter,as shown in Fig.12a.the low air change rate and the relatively high indoor temperature caused the highest formaldehyde concentration in winter in the north.Conversely,in the south,a large percentage of open windows all the year(Fig.12b)made little difference in air change rates among seasons,and the indoor formaldehyde concentrations were mainly determined by temperature。
From the seasonal trends in the north and the south,it can be seen that the indoor temperature,humidity,and ventilation rate were the key factors that affected the seasonal variations of indoor formaldehyde.the higher the indoor temperature and the less the ventilation in the season,the higher the indoor formaldehyde concentration and the over-standard rate were,and vice versa,no matter in the north or the south.Interestingly,the trend in the south was more affected by natural factors(temperature and humidity),whereas the trend in the north was more affected by human factors(heating supplies and window opening behaviors)。
5.4 Influence of room type
Precisely,room type(room function)itself had no impact on the indoor formaldehyde concentration.Nevertheless,the room's type or assigned function affected the decoration load,properties of furnishing materials,and ventilation rate,which affected the formaldehyde concentration.the formaldehyde emission properties of furnishing materials were the most important factor affecting indoor formaldehyde concentration
[114]。 Furnishing materials with a property of easy releasing formaldehyde emission(UF resin,artificial boards,wood floor,etc.)would cause higher formaldehyde concentrations than more environmentally friendly materials(PF resin,solid boards,tile floor,etc.)
[15,77,115,116]。 Additionally,higher decoration load would also lead to higher formaldehyde concentration,given that the properties of furnishing materials were identical
[117]。 the decoration loads,properties of furnishing materials,and ventilation rates varied among rooms,so as The formaldehyde concentrations and over-standard rates as shown in Fig.13。
the average concentrations changed in the order of storage room≫toilet≈bedroom>study>basement≈kitchen>living room>children's room,where master bedroom>secondary bedroom in the bedroom type(Fig.13).storage room,toilet,bedroom,and study were among the room types with high concentrations because of their high decoration loads.Additionally,the storage room was kept closed with little ventilation,resulting in formaldehyde accumulation and thus a much higher concentration.children's rooms also had a relatively high decoration load;however,usually,more environment-friendly materials and furniture were used because of the tendency to protect children,so the concentration was the lowest of all.the large space,high ventilation rate,and commonly tile floor used in the living room resulted in a relatively low concentration.It can be stated that rooms with high decoration load,furnishing materials with high formaldehyde emission,and less ventilation would have high concentrations。
the distributions of the highest concentrations over room type were similar to that of average concentrations with some deviations and followed the order of storage room≫toilet>bedroom≈study>living room>kitchen>children's room>basement.Generally,higher over-standard rates appeared with higher concentrations although deviations existed for basement and living room(Fig.13),and the over-standard rates followed the order of basement>storage room>toilet>bedroom>study>living room>kitchen>children's room.Except in the toilet and basement,the lowest concentrations were within the national standard limit。
Fortunately,note that both the average concentration and over-standard rate in children's rooms were the lowest in all types of rooms,indicating that people already had good environmental awareness,especially for the protection of children。
5.5 Influence of time after decoration
the long-term decay patterns of indoor formaldehyde concentrations and over-standard rates were obtained by plotting the measured data with time after decoration,as presented in Fig.14.Although varying seasonally,the concentrations decreased gradually in the long term under the normal life activities of residents.This was mainly due to the reduced emission rate of formaldehyde from furnishing materials.Both empirical models and diffusion mass transfer-based models showed that the emission rates of VOCs from furnishing materials decreased exponentially with time and thus the concentrations of VOCs.Such a decay pattern was commonly observed by other researchers
[118,119][120⇓~122][30,31,37,51,53,54,59,78]。 Ye et al.analyzed the indoor formaldehyde concentrations from over 2000 dwellings in 22 cities in China and found the concentrations decayed with the time after decoration,although the data were collected from different researchers
[13]。
It seems the average concentration could fulfill the requirement of GB/T 18883-2002 after 18 months of decoration(Fig.14a),which differed from Ye's 12 months
[13]。 However,the results indicate that we had better let the house be ventilated for a period of time after decoration before moving in.Frustratingly,even after 3 years,the concentrations were still much higher than the chronic limit of 0.009~0.01 mg/m
3recommended by CA OEHHA or ATSDR
[13]。 The lowest and highest concentrations also decayed with time in the long term,but a huge difference existed in specific values.the lowest concentrations were reduced to the national standard limit 3 months after decoration(Fig.14b),whereas the highest concentrations still exceeded the limit even after 3 years of decoration(Fig.14c).The over-standard rates kept high within 6 months and decreased rapidly from then on in general(Fig.14d)。
It should be stated that the periods after decoration in Fig.14 concentrated within 12 months,only a little was longer than 12 months,so the decay pattern after 12 months was More likely the extrapolation of the pattern before 12 months.more data with time longer than a year after decoration is needed to confirm the long-term decay pattern after 1~2 years。
6 Limitations
the indoor formaldehyde concentrations of more than 10394 dwellings with 27384 rooms from 62 cities in 31 provinces in China were analyzed in this work.The collected dataset covers all climate zones in China,all seasons,all types of rooms,and a long period ranging from 2003 to 2018.It allows a multidimensional analysis of The change patterns of indoor formaldehyde,which is more comprehensive than previous reviews.However,limitations of this work exist。
One limitation is that the samples were imbalanced among provinces,years,seasons,and different types of rooms,leading to deviations of some details from the actual situation.Nevertheless,the general trends would not change。
Another limitation is that the information on influencing factors was very scarce,such as the indoor temperature and humidity at the sampling date,days after decoration,decoration loads in rooms,properties of furnishing materials,and air change rates.Therefore,full regression with respect to all these factors could not be obtained.This is mainly because only a few researchers recorded the information on these factors in their studies.Furthermore,many recorded factors could not be linked to other factors;for instance,the recorded temperatures have not specified the date and humidity.It is highly suggested to record such information while monitoring。
7 Conclusions
Conclusions can be drawn on the basis of the above results:
(1)Indoor formaldehyde pollution is very severe across China,as indicated by the indoor formaldehyde distribution maps.The average concentrations in 2003~2018 exceeded the national standard limit of 0.1 mg/m3,and the over-standard rates were more than 50%in most years 。
(2)Indoor formaldehyde concentrations decreased gradually after the enactment of national standard GB/T 18883-2002 because of its restriction;however,the concentrations increased from 2013,which deserves our attention.Additionally,a regional discrepancy in change patterns was observed recently,and more work is required to uncover the underlying reasons。
(3)formaldehyde emission properties of furnishing materials,decoration load,indoor temperature and humidity,time after decoration,and ventilation rate(air change rate)are the major factors that influence the indoor formaldehyde concentration.Generally,indoor formaldehyde concentration positively correlates with indoor temperature and decoration load while negatively correlates with time after decoration and air change rate.the humidity should not be isolated from temperature when considering its effect on indoor formaldehyde.materials that easily release formaldehyde would cause higher formaldehyde concentrations,whereas More environment-friendly materials would give lower formaldehyde concentrations.However,the research on the relationship between indoor formaldehyde concentration and properties of furnishing materials and decoration load is relatively rare.more studies are needed to provide references for choosing appropriate furnishing materials and decoration load。
(4)different seasonal trends of indoor temperatures and air change rates between the north and the south,resulting from different climates and heating supply policies,caused different seasonal trends of indoor formaldehyde concentrations between the two zones.the trend was winter>summer>autumn>spring in the north,while it was summer>autumn>spring>winter in the south.Seasons with higher indoor temperatures and lower air change rates had higher concentrations and over-standard rates,whether in the north or in the south.Interestingly,the seasonal trend in the south was more affected by natural factors(temperature and humidity),whereas the trend in the north was more affected by human factors(heating supplies and window opening behaviors)。
Project for laboratory work of Nanjing University of Posts and Telecommunications(2020XSG11)
Jiangsu Professional Brand Construction Project of ‘Polymer materials and engineering'(PZ03019007)
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