Temporal variations in the concentrations of PCDD/Fs and dl-PCBs in ambient air at Da Nang airport, Vietnam, and health risk assessments

doi:10.21203/rs.3.rs-4267074/v1

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Temporal variations in the concentrations of PCDD/Fs and dl-PCBs in ambient air at Da Nang airport, Vietnam, and health risk assessments

https://doi.org/10.21203/rs.3.rs-4267074/v1

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Air pollution by polychlorodibenzo-p-dioxins/polychlorodibenzofurans (PCDD/Fs) and dioxin-like polychlorobiphenyls (dl-PCBs) in three residential areas north, west, and south of Da Nang airport, was determined by using passive air samplers containing polyurethane foam (PUF) discs with three-month sampling intervals from 2017 to 2020. The total toxic equivalents (∑TEQs) of the PCDD/Fs and dl-PCBs, using WHO2005-TEFs, were highest north of the airport (134 to 10610 fg WHO-TEQ/PUF day with an average of 1108 fg WHO-TEQ/PUF day). ∑TEQs were lower west of the airport, between 159 and 381 fg WHO-TEQ/PUF day, and averaged 230 fg WHO-TEQ/PUF day. The lowest ∑TEQs were south of the airport, with a range of 76 and 331 fg WHO-TEQ/PUF day and an average of 152 fg WHO-TEQ/PUF day. Construction activities, including excavation and transportation of dioxin-contaminated soil north of the airport, have increased ambient air PCDD/F and dl-PCB contamination and health risks. The average daily dose values of PCDD/Fs and dl-PCBs through inhalation (ADDA) for residents north of the airport were the highest (10.9 to 3434 fg WHO-TEQ/kg BW/day and average: 597 fg WHO-TEQ/kg BW/day). Residents west of the airport faced lower health risks (13-123 fg WHO-TEQ/kg BW/day and average: 39 fg WHO-TEQ/kg BW/day). Residents south of the airport were exposed to a minimum of 6.2-107 fg WHO-TEQ/kg BW/day, with an average of 28 fg WHO-TEQ/kg BW/day. The maximum and average ADDA values for residents north of the airport exceeded 10% of the tolerable daily intake (TDI) recommended by the WHO (100-400 fg WHO-TEQ/kg BW/day). In comparison, all ADDA values for residents west and south of the airport were less than and within 10% of the TDI.

PCDD/Fs

dl-PCBs

Passive air sampling

Temporal variation

Inhalation exposure

During the “Ranch Hand” Operation from 1962 to 1971, the Da Nang, Bien Hoa and Phu Cat airports were used to store Agent Orange and other herbicides. Da Nang airport was one of three former dioxin hot spots in Vietnam (Dwernychuk 2005; Stellman et al. 2003; Hatfield Consultants 2009). From 2013 to 2018, Vietnam and the US implemented a project to remediate dioxin-contaminated environments north of Da Nang airport. Approximately 95,000 m³ of soil and sediment with concentrations of polychlorodibenzo-p-dioxins/polychlorodibenzofurans (PCDD/Fs) and dioxin-like polychlorobiphenyls (dl-PCBs) exceeding the threshold of 1000 pg WHO-toxic equivalent (TEQ)/g were excavated for treatment using in-pile thermal desorption (IPTD) technology. Currently, 68,000 m³ of material with concentrations ranging from 150 to 1000 pg WHO-TEQ/g are isolated in landfills south of the airport (USAID 2018).

The Da Nang airport is located in the center of Da Nang City and Central Vietnam. Da Nang City has an area of 1285 km² and a population of 1,220,190 people in 2022. Da Nang has a tropical monsoon climate that is hot and humid. The average temperature is approximately 26 °C. The average humidity is 83.4%. The average rainfall is 2505 mm (General Statistics Office 2022).

The Thanh Khe and Cam Le districts are adjacent to Da Nang airport to the north, west, and south. People residing around the Da Nang airport are at risk of exposure to residual dioxin, especially in the Thanh Khe district north of the airport. At the Da Nang airport, the highest ∑TEQ of PCDD/Fs in soil reached 365,000 pg WHO-TEQ/g. The ∑TEQ in blood ranged from 9.3-1220 pg WHO-TEQ/g lipid, and the highest level was in people directly working in the former Agent Orange storage area (Hatfield Consultants 2009; Minh et al. 2009). PCDD/F concentrations in ambient air between 2013 and 2017 north of the airport ranged from 0.164 to 15.3 pg WHO-TEQ/PUF day (Sau et al. 2021). The concentrations in high-risk foods originating in the airport ranged from 3.8 to 95 pg WHO-TEQ/g wet weight, and those in low-risk foods at local markets ranged from 0.03 to 6.1 pg WHO-TEQ/g wet weight (Tuyet-Hanh et al. 2015). ∑TEQs in wild fish in and around the airport ranged from 0.223 to 3120 pg WHO-TEQ/g wet weight (Hatfield Consultants 2009). ∑TEQs in breast milk from women living in Thanh Khe ranged from 3.73 to 72.34 pg WHO-TEQ/g lipid, with an average value of 14.22 pg WHO-TEQ/g lipid, which was greater than that in areas sprayed with herbicides only in Quang Tri Province (10.9 pg WHO-TEQ/g lipid), and in the unsprayed area in Ha Nam Province (4.09 pg TEQ/g lipid) (Nishijo et al. 2012; Nishijo et al. 2014; Nghi et al. 2016; Tai et al. 2011).

The average daily dose (ADD) values through inhalation in residents around the dioxin remediation site at Da Nang airport were estimated to range from 7 to 820 fg WHO-TEQ/kg BW/day from 2013–2017 (Sau et al. 2021). The ADD through food consumption ranges from 1200 to 148000 fg WHO-TEQ/kg BW/day (Tuyet-Hanh et al. 2015). The combined ADD values resulting from inhalation (Sau et al. 2021) and ingestion of high-risk foods (Tuyet-Hanh et al. 2015) at Da Nang airport exceeded the values of tolerable daily intake (TDI) recommended by the WHO (1000-4000 fg TEQ/kg bw/day) (WHO 1998).

Passive air sampling (PAS) with polyurethane foam (PUF) discs has been applied on a global scale to monitor variations in the concentrations and temporal and spatial trends of PCDD/Fs and dl-PCBs (Holoubek et al. 2011; Fiedler et al. 2023a; Fiedler et al. 2023b; Abad et al. 2022), as well as to assess health risks from their inhalation (Tian et al. 2015; Dziubanek et al. 2017; Francisco et al. 2017; Hao et al. 2017; Sun et al. 2017; Sau 2023). The advantages of PAS are its low cost, simple implementation, and power-free nature, and it can be performed continuously for approximately three months or seasonally. The PAS reflects the average concentration of pollutants at optional sampling intervals and is, therefore, more flexible than active air sampling (Cortes et al. 2014; Gao et al. 2014; Vilavert et al. 2014; Sau 2023).

In a previous study, we used PAS devices to monitor PCDD/Fs from March 2013 to February 2017 and assess dioxin exposure during the implementation of a remediation project at the hot spot north of the Da Nang airport (Sau et al. 2021). In this study, the monitoring objects, including PCDD/Fs and dl-PCBs, were expanded. Also, PAS sites in three areas north, west, and south of the Da Nang airport were developed with the following goals: (1) monitor the temporal variations in PCDD/F and dl-PCB concentrations in ambient air between 2017 and 2020 after the end of the hot spot remediation project and (2) assess health risks due to exposure to residual dioxin through inhalation.

Study areas and sampling

TE-200-PAS devices (Tisch Environmental Inc., USA) with PUF discs 140 mm in diameter and 13 mm in thickness were used as described in previous studies (Harner et al. 2006; Sau et al. 2021). PAS devices were installed in a well-ventilated area at a height of 3 m at three locations, S2, S4 and S5 (Fig. 1).

PAS site S2 is north of Da Nang airport with universal transverse mercator (UTM) 48Q coordinates of 200190 m E and 1778120 m N. It is adjacent to the residential area of the Thanh Khe district (TK residential area). The area around site S2 (S2 area) was excavated to remove all dioxin-contaminated materials exceeding the 1000 pg WHO-TEQ/g threshold (USAID 2018). Currently, the ∑TEQs of PCDD/Fs and dl-PCBs in surface soil in the S2 area are less than 1000 pg WHO-TEQ/g.

PAS site S4 is west of the airport (UTM 48Q: 199250 m E and 1775500 m N) and adjacent to the residential area of the An Khe ward (AK residential area). The highest ∑TEQ in the surface soil around site S4 (S4 area) was 115 pg WHO-TEQ/g (Hatfield Consultants 2009).

PAS site S5 is south of the airport (UTM 48Q: 199500 m E and 1773700 m N) and adjacent to the residential area of the Cam Le district (CL residential area). The average ∑TEQs in the surface soil around site S5 (S5 area) are only 8 pg WHO-TEQ/g. However, in this S5 area, 68,000 m³ of contaminated materials with concentrations ranging from 150 to 1,000 pg/g are buried by closed isolation at the SIA site (Fig. 1) (USAID 2018).

The surface soil in the S2, S4, and S5 areas is not covered, and these are the "open sources" for releasing PCDD/Fs and dl-PCBs into the ambient air. They are hereafter referred to as "open sources" in the S2, S4, and S5 areas.

This study was implemented from November 16, 2016, to December 21, 2020. PAS samples were collected continuously every three months. Details regarding PAS sample identification and deployment intervals in the three study areas are described in Table A1 in the supplementary information.

Standards and chemicals

All organic solvents, reagents, and adsorbents were supplied by Merck (Darmstadt, Germany). The natural and ¹³C-labeled standards of PCDD/Fs and dl-PCBs, and the ³⁷Cl-labeled 2,3,7,8-TCDD clean-up standard were obtained from Cambridge Isotope Laboratories (Massachusetts, USA). Fifteen ¹³C-labeled 2,3,7,8-substituted PCDD/Fs at 2 and 4 ng of each compound were diluted from a stock solution of EDF-8999 as surrogates. The surrogates were spiked on each PUF disc at the beginning of PAS deployment, as described in our previous studies (Sau et al. 2021; Sau 2023).

Analysis of PCDD/Fs and dl-PCBs

Before extraction of the PAS samples, twelve ¹³C-labeled dl-PCBs for quantification of dl-PCBs and two recovery standards (¹³C-labeled 1,2,3,4-TCDD and 1,2,3,7,8,9-HxCDD) for determination of the retention efficiency of surrogates were added to the PUF discs. Subsequently, the ³⁷Cl-labeled 2,3,7,8-TCDD clean-up standard and ¹³C-labeled dl-PCB recovery standards were added to the extract (Sau 2023). Then, the extracts were prepared according to the US EPA method 1613 (US EPA 1994) and the US EPA method 1668 (US EPA 2008).

PCDD/Fs and dl-PCBs were analyzed by high-resolution gas chromatography (HRGC, Agilent 7890A) coupled with high-resolution mass spectrometry (HRMS, AutoSpec Premier, Water). The initial temperature of GC oven was at 150 °C for 2 min. The GC temperature was increased by 20 °C/min until 220 °C and kept at 220 °C for 16 min. Then, the temperature was increased from 220 °C to 320 °C by 5 °C/min and maintained at 320 °C for 15 min. The GC injector temperature was 280 °C. The MS source temperature was 290 °C. The helium was carrier gas at a 1.0 mL/min flow rate. A DB-5MS capillary column (Agilent, USA) with a 60-m length, 0.25-mm internal diameter, and 0.25-µm film thickness was used. The MS operated at a resolution greater than 10,000 with a 10% valley. The PCDD/F and dl-PCB data analyses used the MassLynx V4.1 and the Quan Lynx V4.1 software (Sau 2023).

Quality assurance and quality control

Before PAS deployment, the PAS devices were washed and rinsed several times with double-distilled water, followed by rinsing with acetone and then hexane. The PUF discs were cleaned by Soxhlet extraction in 24 h with each solvent of acetone, hexane, and toluene, respectively. The PUF discs were dried in a vacuum oven at 50 °C for 5 h and stored in amber airtight glass bottles. The field and laboratory blanks and the matrix spiked samples were analyzed similarly to the PAS samples. Each analytical batch included a method blank and a matrix-spiked sample. All analyses were conducted in an accredited laboratory (Bureau of Accreditation 2024).

The retention efficiencies of the ¹³C-labeled PCDD/F surrogates on the PUF discs, including the recoveries of surrogates in the laboratory analysis of the PAS samples, ranged from 23.1% to 130%, and their average values ranged from 37.1% to 103%. The recoveries of the ¹³C-labeled dl-PCBs ranged from 30.6% to 166%, with average values between 74.7% and 108%. PCDD/Fs and dl-PCBs were not detected in the blank samples. The concentrations of PCDD/Fs and dl-PCBs detected in the PAS samples were at least three times greater than the limit of detection (LOD). The signal/noise ratios of the detected congeners were greater than 10. The LOD of PCDD/Fs for the PAS matrix was equivalent to 1 to 25 fg/PUF day, and that of dl-PCBs ranged from 2 to 6 fg/PUF day. The percentage differences in the concentrations of each PCDD/F and dl-PCB congener in the matrix-spiked PAS samples were well within acceptable limits, not exceeding 22%.

Data analysis and statistics

The TEQ values of the PCDD/F and dl-PCB congeners were determined using the WHO₂₀₀₅-TEFs (Van den Berg et al. 2006). Half of the LOD value was calculated for the TEQ of congeners with a concentration less than the LOD. The partial TEQs, including TEQ_D, TEQ_F, TEQ_D/F, and TEQ_PCB were also calculated. Where: the TEQ_D represents the total TEQs of the seven 2,3,7,8-substituted PCDDs; the TEQ_F signifies the total TEQs of the ten 2,3,7,8-substituted PCDFs; the TEQ_D/F denotes the total TEQs of the seventeen 2,3,7,8-substituted PCDD/Fs; and the TEQ_PCB indicates the total TEQs of the twelve dl-PCBs. The ∑TEQs are the sum of the partial TEQs of the PCDD/Fs and dl-PCBs.

Statistical analysis of the PCDD/F and dl-PCB concentrations, partial TEQs, and ∑TEQs was performed using Excel 2019 software, and the data were compared by one-way ANOVA with Tukey post hoc tests. The significance level was p < 0.05.

ADD calculation through inhalation

There are three routes of dioxin exposure, namely, food, inhalation, and dermal contact, to pollution sources, of which approximately 10% of TDI is from inhalation (WHO 1998). Therefore, the WHO-recommended TDI for inhalation will be in the range of 100 to 400 fg WHO-TEQ/kg BW/day.

In this research, we used the following equation (US EPA 2018) to calculate the ADD values through inhalation (ADD_A, fg WHO-TEQ/kg BW/day), similar to our previous studies (Sau et al. 2021; Sau 2023):

ADD_A = (C_A × InhR × ET × EF × ED) / (BW × AT)

Where: C_A is the ∑TEQs of the PCDD/Fs and dl-PCBs in ambient air (fg WHO-TEQ/m³) at each PAS site, which was converted from the average daily concentrations in the PAS samples (fg WHO-TEQ/PUF day) at the 3.5 m³/day average sampling rate (Mari et al. 2008; Li et al. 2011; Moussaoui et al. 2012; Nizzetto et al. 2020; Sau et al. 2021; Sau 2023); InhR refers to the air inhalation rate by age group (m³/day); ET represents the exposure time in a day (h/day); EF denotes the exposure frequency in a year (days/year); ED indicates the exposure duration by year (years); BW is the body weight of residents by age group (kg); and AT represents the average time by day (days) (US EPA 2018).

The details of parameters for ADD_A calculation are thoughtfully presented in the supplementary information.

The PCDD/Fs and dl-PCBs, found in ambient air at Da Nang airport, originated from the emission of soil dust, which was contaminated with residual Agent Orange/dioxin (Sau et al. 2021). In this study, we consulted the dominant wind directions in Da Nang City during the dry and rainy seasons (USAID 2010) to select the location to install PAS devices in the center of each dioxin-contaminated area and adjacent to the residential area to ensure representativeness and reflect the current air pollution situation in that area. The 48 PAS samples, including 16 samples from each area (Table A1 in the supplementary information), were collected and analyzed.

Temporal variations in concentrations of PCDD/Fs and dl-PCBs in ambient air

The PCDD/F and dl-PCB concentrations in the PAS samples, ∑TEQs, and their partial TEQs, which were recorded from 2017 to 2020, are summarized in Table 1. ∑TEQs and the partial TEQs between 2017 and 2020 are illustrated in Fig. 2.

Table 1 Concentrations of PCDD/Fs and dl-PCBs (fg/PUF day), ∑TEQs and partial TEQs (fg WHO-TEQ/PUF day) in ambient air at Da Nang airport between 2017 and 2020

PAS site	S2 (n = 16)			S4 (n = 16)			S5 (n = 16)
Congener	Min.	Mean	Max.	Min.	Mean	Max.	Min.	Mean	Max.
2378-TCDD	27	938	10481	21	59	194	10	40	123
12378-PeCDD	22	46	79	26	42	72	9	24	51
123478-HxCDD	10	30	119	14	28	77	0	16	42
123678-HxCDD	30	85	265	34	81	264	15	47	168
123789-HxCDD	23	55	135	29	63	184	12	45	172
1234678-HpCDD	169	474	992	187	749	3269	89	476	2450
OCDD	492	3134	15434	666	3195	13141	422	1737	6668
2378-TCDF	81	139	254	99	134	207	52	92	198
12378-PeCDF	38	71	135	53	75	112	30	52	110
23478-PeCDF	56	135	231	88	129	184	47	83	157
123478-HxCDF	49	94	177	70	99	140	38	71	201
123678-HxCDF	33	77	130	42	75	130	34	59	108
234678-HxCDF	33	85	165	52	87	158	33	63	109
123789-HxCDF	13	62	345	16	48	110	14	45	171
1234678-HpCDF	117	216	377	131	236	376	106	230	740
1234789-HpCDF	23	80	505	14	63	166	19	66	277
OCDF	25	68	326	28	106	497	13	71	184
PCB#81	95	223	451	65	198	418	31	190	549
PCB#77	1162	3085	8738	1452	2464	4896	0	1973	5888
PCB#123	218	507	1386	191	468	945	124	454	1739
PCB#118	6096	13680	41682	7323	14889	24981	4840	12862	34802
PCB#114	193	562	1478	307	657	1055	192	473	1350
PCB#105	2174	6008	16540	2474	5801	10157	2684	5319	11622
PCB#126	89	142	248	62	157	338	65	120	222
PCB#167	183	426	1278	246	448	778	146	421	1211
PCB#156	452	929	2944	517	1024	1918	317	894	2505
PCB#157	110	266	811	154	302	712	80	330	1204
PCB#169	12	36	193	11	33	94	0	28	208
PCB#189	34	81	189	25	87	197	9	85	263
∑PCDDs	877	4761	21010	1064	4219	15817	622	2385	9674
∑PCDFs	491	1027	2451	693	1049	1680	446	826	1942
∑dl-PCBs	11550	25940	74020	13402	26523	43522	9922	23149	60723
TEQ_D	78	1006	10560	62	127	264	26	80	239
TEQ_F	41	91	173	62	88	130	36	62	106
TEQ_D/F	122	1093	10590	145	213	360	68	138	321
TEQ_PCB	10	15	20	7	17	37	4	13	28
∑TEQs	134	1108	10610	159	230	381	76	152	331

min. minimum value, mean average value, max. maximum value

Table 1 and Fig. 2 show that at site S2 north of the airport (Fig. 1), ∑TEQs ranged from 134 to 10610 fg WHO-TEQ/PUF day and averaged 1108 fg WHO-TEQ/PUF day. ∑PCDDs fluctuated in the range of 877-21010 fg/PUF day, with an average value of 4761 fg/PUF day. ∑PCDFs were 491-2451 fg/PUF day, and the average value was 1027 fg/PUF day. ∑PCBs ranged from 11550 to 74020 fg/PUF day, with an average concentration of 25940 fg/PUF day.

At site S4 west of the airport (Fig. 1), the following air pollution levels were recorded (Table 1, Fig. 2): ∑TEQs ranged from 159 to 381 fg WHO-TEQ/PUF day, and the average was 230 fg WHO-TEQ/PUF day. ∑PCDDs were in the range of 1064-15817 fg/PUF day, with an average of 4219 fg/PUF day. ∑PCDFs were 693-1680 fg/PUF day, and the average was 1049 fg/PUF day. ∑PCBs were between 13402 and 43522 fg/PUF day, averaging 26523 pg/PUF day.

At site S5 south of the airport (Fig. 1), the ∑TEQs ranged from 76 to 331 fg WHO-TEQ/PUF day, with an average of 152 fg WHO-TEQ/PUF day (Table 1, Fig. 2). ∑PCDDs ranged from 622 to 9674 fg/PUF day, with an average of 2385 fg/PUF day. ∑PCDFs ranged between 446 and 1942 fg/PUF days, with an average value of 826 fg/PUF days. ∑dl-PCBs were in the range of 9922-60723 fg/PUF day, and the average was 23149 fg/PUF day.

A comparison of the ∑TEQs in ambient air at sites S2, S4, and S5 with the contamination levels of PCDD/Fs and dl-PCBs in the soil of the “open sources” in the respective areas revealed a positive correlation. The open source in the S2 area had the highest ∑TEQs (up to 1000 pg WHO-TEQ/g in surface soil), and in 2017, aircraft taxiway E7 was constructed, which increased the dispersion of dust; therefore, the ∑TEQs in ambient air were the highest (ranging from 134 to 10610 fg/PUF day and averaging 1108 fg/PUF day). Similarly, when the open source in the S4 area was lower (115 pg WHO-TEQ/g in surface soil), the ∑TEQs in ambient air at site S4 were also much lower (from 159-381 fg/PUF day and average: 230 fg/PUF day). The open source in the S5 area was the lowest (8 pg WHO-TEQ/g in surface soil), so the ∑TEQs in ambient air at site S5 were also the lowest (between 76-331 fg/PUF day and average: 152 fg/PUF day). Compared to those at site S2, which was also an open-source site, the ∑TEQs in ambient air at site S4 were 4.8 times lower, and those at site S5 were 7.3 times lower. The ∑TEQs in ambient air at site S5 were 1.5 times lower than those at site S4.

When comparing the open-source S2 area with the closed-source SIA in the S5 area, although the contaminated materials had the same concentration of 1000 pg/g, because SIA is a closed and sealed source, there was no dispersion of PCDD/Fs and dl-PCBs from SIA into the ambient air. This indicates that the open-source status has been directly linked to the release of PCDD/Fs and dl-PCBs into the air. The increase in air pollution caused by PCDD/Fs and dl-PCBs depends mainly on the level of dioxin residue in soil from open sources.

Annual variations in PCDD/F and dl-PCB concentrations in ambient air

The annual variations in PCDD/F and dl-PCB concentrations in ambient air, expressed as ∑TEQs at three sites, S2, S4 and S5, are illustrated in Fig. 3 and summarized in Table A2 in the supplementary information.

Fig. 3 and Table A2 show that at site S2, air pollution by PCDD/Fs and dl-PCBs in 2017 was very high, and ∑TEQs were in the range of 134-10605 fg WHO-TEQ/PUF day, with an annual average value of 3562 fg WHO-TEQ/PUF day (Table A3). Compared to those in 2017, the annual average ∑TEQs in 2018, 2019, and 2020 decreased by 13.2, 12.2, and 17.8 times, respectively. ∑TEQs were unusually high in 2017 because during the period from February 2017 to November 2017, in the S2 area north of Da Nang airport, construction activities on aircraft taxiway E7, including excavating and transporting dioxin-contaminated soil, took place. These activities result in dust emission and increased PCDD/F and dl-PCB pollution in ambient air. Specifically, the dry season was between February and May 2017, which was also the peak of excavation and transport of dioxin-contaminated soil, so the ∑TEQs in ambient air increased to 10605 fg WHO-TEQ/PUF day. Between May and August 2017, when excavation activities were basically completed, the ∑TEQs decreased to 3175 fg on the WHO-TEQ/PUF day. From August to November 2017, the construction of taxiway E7 was completed, and it was also the rainy season, so the ∑TEQs decreased significantly (693 fg WHO-TEQ/PUF day). ∑TEQs then continued to decline to 190 fg WHO-TEQ/PUF day during the next PAS interval between November 2017 and February 2018.

Fig. 3 and Table A2 also show that the annual ∑TEQs in ambient air were highest at site S2, much lower at site S4, and lowest at site S5. The annual average ∑TEQs in the period from 2017–2020 at site S2 (205–3652 fg WHO-TEQ/PUF day) were greater than those at site S4 (188–299 fg WHO-TEQ/PUF day) and site S5 (102–221 fg WHO-TEQ/PUF day). ∑TEQs at all three sites generally decreased annually because, in the years 2018-2020, no construction or excavation activities occurred in those areas.

Compared with the values of ∑TEQs in ambient air at site S2 recorded during environmental remediation activities using IPTD technology in the period 2013-2017 (∑TEQs: 151-2405 fg WHO-TEQ/PUF day, average: 714 fg WHO-TEQ/PUF day) (Sau et al. 2021), there was a more significant increase in PCDD/F and dl-PCB contamination between 2017 and 2020. This is because the dioxin-contaminated soil surface in the S2 area was not covered during the construction of aircraft taxiway E7. Specifically, the maximum ∑TEQs in this study were up to 4.4 times greater, with the average ∑TEQs being 1.6 times greater.

At site S4, the ∑TEQs in ambient air in 2017 ranged from 161-381 fg WHO-TEQ/PUF day, with an average value of 299 fg WHO-TEQ/PUF day (Tables A2 and A3). The annual average ∑TEQs for the three consecutive years, 2018, 2019, and 2020, decreased by 1.3, 1.5, and 1.6 times, respectively.

At site S5, the ∑TEQs in 2017 ranged from 117-331 fg WHO-TEQ/PUF day and averaged 221 fg WHO-TEQ/PUF day. The annual average ∑TEQs over the next three years - 2018, 2019, and 2020 - decreased by 1.4, 1.7, and 2.2 times, respectively.

Seasonal variations in the PCDD/F and dl-PCB concentrations

Da Nang City's climate has two seasons: dry season and rainy season. The dry season is usually from mid-February to early September. The rainy season is from mid-September to late January of the following year (USAID 2010). Fig. 4 shows the seasonal variations in concentrations of the PCDD/Fs and dl-PCBs through the ∑TEQs values at three sites, S2, S4, and S5.

Fig. 4 shows that at site S2, the ∑TEQs in the dry season ranged from 183-10605 fg/day PUF, with an average of 1961 fg/PUF day, and the average ∑TEQ value was 7.7 times greater than that in the rainy season (134-693 fg/PUF day, average: 255 fg/PUF day). At site S4, the ∑TEQs in the dry season (159-381 fg/PUF day, average: 236 fg/PUF day), with an average value, were 1.1 times greater than the ∑TEQs in the rainy season (161-323 fg/PUF day, average: 224 fg/PUF day). At site S5, the ∑TEQs in the dry season ranged from 76-331 fg/PUF day, with an average of 163 fg/PUF day, and the average ∑TEQ was 1.2 times greater than the ∑TEQ in the rainy season (87-231 fg/PUF day, average: 140 fg/PUF day).

The average ∑TEQs during the dry season at site S2 were 8.3 times greater than those at site S4 and 12 times greater than those at site S5. However, the average ∑TEQs during the rainy season at site S2 were only 1.1 times greater than those at site S4 and 1.8 times greater than those at site S5. This indicates that residents of the S2 area are strongly affected by PCDD/F and PCB pollution during the dry season.

Assessment of health risks from inhalation exposure to the PCDD/Fs and dl-PCBs

A health risk assessment of inhalation exposure to PCDD/Fs and dl-PCBs between 2017 and 2020 through ADD_A values in three residential areas, TK, AK, and CL, is illustrated in Fig. 5. The details of the parameters and ADD_A calculation results are presented in Table A3 in the supplementary information.

Fig. 5 and Table A3 show that for the residents, including adults and children, in the TK area around site S2, the minimum ADD_A (10.9-43.4 fg WHO-TEQ/kg BW/day) was lower than the lower threshold of 10% TDI recommended by the WHO (100 fg WHO-TEQ/kg BW/day). The average ADD_Avalues(90.4-359 fg WHO-TEQ/kg BW/day) were within 10% of the TDI (100-400 fg WHO-TEQ/kg BW/day). The maximum ADD_A (865-3434 fg WHO-TEQ/kg BW/day) exceeded the upper threshold of 10% TDI (400 fg WHO-TEQ/kg BW/day) by 2.2 to 8.6 times.

Our previous study between 2013 and 2017 (Sau et al. 2021) showed that residents in the same S2 area had ADD_A values in the range of 11.3-780 fg WHO-TEQ/kg BW/day and an average of 57.6-228 fg WHO-TEQ/kg BW/day. Thus, the average health risk due to exposure to PCDD/Fs and dl-PCBs through inhalation in this study between 2017 and 2020 was 3.6 times greater than that in 2013–2017. This shows that although the dioxin concentration in the soil north of the airport is below the treatment threshold of 1000 pg/g, the risk of exposure to PCDD/Fs and dl-PCBs through inhalation is still high, especially during construction activities.

For the residents in the AK area surrounding site S4, the minimum ADD_A values (13-51.5 fg WHO-TEQ/kg BW/day) and the average ADD_A values (18.8-74.4 fg WHO-TEQ/kg BW/day) were lower than the threshold of 10% TDI (Fig. 5). The maximum ADD_A values (31.1-123 fg WHO-TEQ/kg BW/day) were lower and within the range of 10% TDI.

Residents in the CL area around site S5 had very low ADD_A. The minimum ADD_A values (6.2-24.6 fg WHO-TEQ/kg BW/day) and average ADD_A values (12.4-49.2 fg WHO-TEQ/kg BW/day) were both lower than the lower threshold of 10% TDI (Fig. 5). The maximum ADD_A values (27-107 fg WHO-TEQ/kg BW/day) were all lower and within the 10% TDI.

Fig. 5 also shows that during the same period of 2017–2020, residents in the TK area, which is around the former dioxin hot spot north of Da Nang airport, still suffered health risks due to inhalation exposure to PCDD/Fs and dl-PCBs that were 15–21 times greater than those in the AK area west of the airport and the CL area south of the airport.

Temporal variations in the PCDD/F and dl-PCB concentrations in ambient air around the Da Nang airport were evaluated between 2017 and 2020. The highest ∑TEQs, from 134 to 10610 fg/PUF day, were recorded north of the airport. Air pollution was much lower west of the airport (159-381 fg/PUF day) than north and lowest south of the airport (76-331 fg/PUF day). Uncovered dioxin-contaminated soil has become an open source for PCDD/F and dl-PCB emissions into ambient air. Extremely high concentrations of PCDD/Fs and dl-PCB in ambient air were recorded during the construction and excavation activities of contaminated materials.

Residents of the TK area north of the airport were exposed to high levels from inhalation (10.9-3434 fg WHO-TEQ/kg BW/day) and exceeded the upper threshold of 10% TDI recommended by the WHO. Dioxin exposures in the western AK area (13-123 fg WHO-TEQ/kg BW/day) and the southern CL area (6.2-107 fg WHO-TEQ/kg BW/day) were minimal and remained below and within the 10% TDI threshold.

Although the dioxin hot spot in Da Nang airport has been treated, it still poses a high risk to human health. It is necessary to apply measures to limit dust dispersion, strictly control construction activities, and continue to monitor environmental quality to manage and minimize health risks.

Acknowledgments

The Vietnamese side of the Joint Vietnam-Russia Tropical Science and Technology Research Center supported this study. The authors thank the Joint Vietnam-Russia Tropical Science and Technology Research Center for supporting and encouraging this research.

Acknowledgments

The authors thank the Joint Vietnam-Russia Tropical Science and Technology Research Center for supporting and encouraging this study.

Author contributions

TKS designed the study, implemented the installation of PAS devices, collected PAS samples, conducted the experiments, analyzed the data, wrote and finalized the manuscript, and approved it for submission.

Funding

This study was supported by the Vietnamese side of the Joint Vietnam–Russia Tropical Science and Technology Research Center.

Data availability

The data of this research is available from the corresponding author.

Ethics approval and consent to participate

Not applicable.

Consent for publication

The authors agree to publish this article under the Environmental Science and Pollution Research.

Competing interests

The authors declare no competing interests.

Conflicts of interest

The authors declare no the potential conflicts of interest concerning this article's study, authorship, and publication.

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Temporal variations in the concentrations of PCDD/Fs and dl-PCBs in ambient air at Da Nang airport, Vietnam, and health risk assessments

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