The distribution of people's age and residence time of the two groups were similar, consistent with the selection criteria and priority survey of subjects with a long, stable residence time in two areas TK and ST in order to eliminate the confounding factor of time. Residents aged 45–69 had a high percentage in both groups (TK group: 79.60% and ST group: 68.75%) (Table 1). The percentage of residents living for more than 15 years was the majority (89.30% in TK group and 90.75% in ST group) (Table 1). As such, these people had settled down, had long-term exposure to AO/dioxin, and bore many health risks due to daily exposure to the surroundings of the hot spot (Hatfield Consultants and Office of the National Steering Committee 2009), local high-risk air and food use (Sau et al. 2021; Tuyet-Hanh et al. 2015).
In this study, women were prioritized for interviews in both groups (Table 1), so they accounted for the majority (TK group: 70.15%, ST group: 67.50%). Because women were subjects of reproductive health and could answer extensively about diseases and illnesses of family members. Women also often did housework, so they had more time to live and work at home in the TK group and ST group regions more often and for a long time. Women also worked in jobs with low exposure to toxic elements related to industrial and residential activities. Women were also less likely to use alcohol, beer, and tobacco than men. Thus, this study limited confounding factors and excluded health risks, not due to dioxin exposure.
Exposure to risk factors for diseases such as foodstuffs, waste treatment, waste incineration, pesticides, chemical fertilizers, cement, metallurgy, rubber and wood processing, etc. may also increase the risks of diseases (Minh et al. 2019). Table 2 shows that the proportion of people exposed to toxic factors not related to dioxin in both TK and ST groups was very low, only from 0–0.75%, therefore, in the assessment of this study, interference from industrial and residential activities can be excluded.
The proportion of people who had been directly and indirectly exposed to AO/dioxin during the war and stored in Da Nang airport of the TK group was 3.48% and 9.45%, respectively, which was higher than that of the ST group (0% and 1.75% respectively) (Table 2). But the greatest health risk and high risk of AO/dioxin exposure were for people in the TK group due to their frequent and long-term residence around former hot spot locations. This was the main cause of the difference in disease patterns between the TK group and the ST group.
There were still about 13.7% of households in the TK group that self-increasing, growing vegetables, raising fish and poultry around the airport and using it as food, which was the cause of food poisoning with the rate of 13.7% and higher than that of the ST group (Table 3). This shows that the risk of AO/dioxin exposure and health risks in the TK group was present and people in the TK group suffered from many diseases at a higher rate than in the ST group. This was also consistent with previous results (Wittsiepe et al. 2007; Todaka et al. 2010).
Previous studies have shown that disease properties are related to sex and age (Choi et al. 2017; Nordström et al. 2016). This study investigated and compared the disease pattern according to age and sex of both TK and ST groups, focusing on the diseases related to a number of systems and organs in the body that are sensitive to AO/dioxin. Dioxin exposure has damaged a number of organ systems in the human body such as the liver, nervous system, immune system, hormones, respiratory system, lungs, etc. (Center for Health Environment and Justice 1999).
The results in Tables 4, 5, 6, and 7 indicated that dioxin exposure by residents near the hot spot increased the prevalence of multiple diseases. The prevalence of diseases in the whole group aged 18–69 was 1.1 to 9.6 times higher than that of the ST group (Table 4); those aged 45–69 was 1.1 to 12 times higher than the ST group (Table 5); Between the ages of 18–44, the prevalence of hypertension, stomach, joints, ear-nose-throat, kidney-urinary system, eyes, diabetes, and hypotension was 1.4 to 13.7 times higher than ST group.
The disease pattern depends on sex, but dioxin exposure due to residence near the hot spot was still the main factor creating the difference in disease pattern between the two groups of TK and ST (Table 6). Health risks and disease risks were higher for both sexes of the TK group. Women in the TK group had a higher rate of all diseases than the ST group from 1.1 to 17.3 times and for men, it was 1.1 to 3.6 times higher (except for diabetes and diseases of the nervous system). Compared with the ST group by two ages in Tables 7 and 8, it was found that the prevalence of most diseases in men in the TK group was from 1.7 times to 8.4 times higher in the age group 45–69 and higher from 1.2 times to 3.7 times in the age group of 18–44; and these rates in women of the TK group were from 1.2 times to 12 times higher and from 1.4 times to 5 times higher in the two age groups, respectively.
The dioxin concentrations in breast milk collecting in 2008 and 2009 from mothers of the TK group were high and ranged from 12.8 to 14.3 pg TEQ/g lipid (Tai et al. 2011). Dioxin exposure from the milk of mothers living near the Da Nang hot spot in the TK group had a negative impact on the physical, nervous system, cognitive, language, autistic traits, and motor development of the children (Nishijo et al. 2012; Anh et al. 2014; Nishijo et al. 2014; Tran et al. 2016).
The results of this study on the effects of AO/dioxin on reproductive health also noted the state of reproductive complications in women as well as the risk of birth defects in young children (Institute of Medicine, 2014). However, no difference was found in the mean age of puberty and the proportion of women with menstrual disorders. Table 10 shows that women of the TK group residing around Da Nang airport were at risk of a higher number of reproductive complications and at a higher rate than the ST group residing far from the airport. In the TK group, women aged 45–69 had a longer exposure to AO/dioxin than women aged 18–44, so they suffered many reproductive complications such as dead malformed birth, living malformed birth, stillbirth, and postnatal death. And especially, only between the ages of 45–69 had children born with delayed physical development and mental retardation (Nishijo et al. 2015; Pham et al. 2015; Tai et al.2016).