| Formaldehyde is one of the most important carcinogens.There paid more attention to building materials,which regarded as release sources indoor,in previous studies,but lack of studies on outdoor sources and natural gas combustion sources.We measured both outdoor and indoor formaldehyde concentrations for over one year in Nanjing（east-central China）from April,2019 to January,2021.The outdoor/indoor（O/I）ratio was calculated.At the same time,the emission characteristics of formaldehyde produced by natural gas combustion were measured in 9 kitchens in Nanjing.As a result,64.8%of the measured outdoor concentration have exceeded the chronic reference exposure criteria of 0.009 mg/m3set by Office of Environmental Health Hazard Assessment（OEHHA）.The outdoor concentration was highest in summer with median value of 0.020 mg/m3and lowest in spring with median value of 0.009 mg/m3.Diurnally,outdoor formaldehyde concentration was highest at noon with median value of 0.013 mg/m3and lowest at night with median value of 0.01 mg/m3.Health analysis revealed that cancer risk by exposure to this concentration level is 1.6×10-4,higher than threshold limit of 10-6.In order to verify the reliability and extensibility of the samples in this study,we took samples for 48hours respectively from different citys,such as Nanjing,Hangzhou,Shanghai,Chongqing,Guangzhou and Beijing.In the six cities,the concentrations of formaldehyde in Nanjing,Hangzhou,Guangzhou and Beijing were relatively high（median values were 10.7μg/m3,10.0μg/m3,11.6μg/m3and 9.2μg/m3,respectively）.Concentrations in Shanghai and Chongqing were relatively low（median 7.9μg/m3and 6.1μg/m3,respectively）.In terms of time distribution,the peak point in a day occurs between 12:00 and 15:00.There are also peak points in 8:00-10:00 in the morning and 17:00-19:00 in the evening,but the degree and time of occurrence are not uniform.According to the computational model used to assess cancer risk from formaldehyde exposure,the lifetime increased risk of cancer due to outdoor formaldehyde concentration was 1.6×10-4(quartile:1.1×10-4,three-quartile:2.7×10-4).EPA recommends that the acceptable lifetime carcinogenic risk of a compound be 10-6 to 10-4,with the results based on outdoor formaldehyde concentrations above this threshold.It suggests that exposure to outdoor formaldehyde poses a serious risk of cancer.In different cities,except Chongqing,the lifetime cancer risk of outdoor formaldehyde in five other cities exceeded the acceptable lifetime risk threshold of compounds proposed by EPA.It can be seen that the risk of outdoor formaldehyde exposure can not be ignored in the whole country,rather than limited to Nanjing.In addition,the median O/I ratios are 0.18 and 0.27 in two offices,indicating that outdoor formaldehyde contributes to indoor concentrations by about one quarter.The change of O/I ratio also shows a similar seasonal and diurnal trend as outdoor concentrations（highest in the summer in a year and at noon in a day）.Outdoor formaldehyde concentration is therefore not negligible as a contributor to indoor concentration,especially as indoor concentration limit gets continuously lowered.This factor should be taken into account in indoor air quality design and maintenance.The experimental results of natural gas confirm that the combustion of natural gas will produce a large amount of formaldehyde,which means a high risk to people’s health.Gas emission coefficients are mainly in the range of 50-200 mg＿HCHO/m3＿natural gas（median 76 mg/m3）.The emission coefficient decreases with the increase of gas velocity.The results confirm that the combustion of natural gas produces a large amount of formaldehyde,which poses a risk to people’s health.At the same time,we studied the regional movement characteristics of formaldehyde released by natural gas combustion.The study found that under the condition of open kitchen door,formaldehyde produced in the kitchen quickly dispersed to the living room for there is considerable air exchange between the two areas.We studied the influence of range hood on natural gas combustion.Under different combustion densities of natural gas,there have been remarkable effect of indoor air control strategies. |
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