For URTI, the result of this study identified that PM10, SO2, NO2 and CO have strong immediate and lag cumulative effects in the females, adolescents, and elderly. PM2.5 has lag effects but has no significant lag cumulative impact effects on gender and age.
Original studies were rare for URTI. A study from Zhang Ying12 demonstrated that the influence of air pollution on URTI was more obvious than lower respiratory tract infections. Findings of this study implied that it’s necessary to explore the association between URTI and air pollutions with more time series studies. The current study investigated the relationship between ambient air pollutants and outpatient visits of URTI in Zhengzhou, China, from October 28, 2013 and May 1, 2018. We found that average annual PM2.5 (83 µg/m3), PM10 (147 µg/m3), and NO2 (52 µg/m3) concentrations were higher than the ambient air quality secondary standard (GB3095-2012) in china, and PM2.5, PM10, SO2, NO2, CO were positive related to clinical visits for URTI, especially NO2 (r = 0.362, P < 0.01). For each 10 µg/m3 increase, PM10, SO2, NO2 had the maximum effects at lag day 0 [RRPM10: 1.0011, 95%CI: (1.0002–1.0020); RRSO2: 1.0084, 95%CI: (1.0039–1.0130); RRNO2: 1.0149, 95%CI: (1.0111–1.0188), respectively], while PM2.5 and CO had highest risks on lag 15 day [RRPM2.5: 1.0014, 95%CI: (1.0003–1.0025); RRCO: 1.1851, 95%CI: (1.0525–1.3344), respectively]. The risks of O3 didn’t show statistical significance.
This research is consistent with other parts of China, such as Beijing, Shenyang, Lanzhou, etc. Especially, in Shenyang28, which reported that exposure to ambient PM2.5, PM10, SO2, NO2, CO had statistical significant relations with outpatient visits for URTI, within the optimal lag period, when concentrations of PM10, PM2.5, SO2, NO2, CO increased each 10 µg/m3 [RRPM10 1.002, 95%CI: (1.0001–1.0004); RRPM2.5 1.0014, 95%CI: (1.0000–1.0030); RRSO2: 1.0032, 95%CI: (1.0017–1.0047); RRNO2: 1.0086, 95%CI: (1.0037–1.0130); RRCO: 1.1900, 95%CI: (1.0879–1.3212), respectively)]. O3 exposure was reported positive associated with URTI in some reports29,30, but not all previous studies. Our study found that increased O3 level was unrelated with outpatient visits of URTI. Probable reasons for this difference may include different O3 concentrations along with different climate temperatures in different regions. The O3 pollution was more obvious in the summer days.
Alternatively, O3 and other air pollutants interactions may result in no significant impact on outpatients with URTI14. Similarly, a study conducted in Beijing, found that the increases in concentrations of PM10, SO2, and NO2 by per 10 µg/m3 had significant impacts on daily URTI outpatient visitors [RRPM10: 1.0113, 95%CI: (1.0049–1.0173); RRSO2: 1.0114, 95CI: (1.0077–1.0183); RRNO2: 1.0213, 95%CI: (1.0150–1.0213), respectively)]. The RR-values were higher in Zhengzhou than those reported in Shenyang, but lower than Beijing. Moreover, study indicated that different pollutants in different areas had different strongest-effect-lag-time for the same disease and their corresponding RR-values, which might be considered to the human susceptibility and regional pollutants with significant regional characteristics31. More studies have been done in the north China than in the south, due to the worse air quality in the north China32, which was contributed by heavy industrial emission and heating season coal consuming, and also related to the local geographical environment, economic development, energy structure, air pollutions control and weather conditions33.
In the past few years, the levels of particulate matters (PM10, PM2.5) in Zhengzhou were exceeding the national secondary maximum contaminant level, and NO2 was 1.3 times higher than the national standard, while SO2, CO, O3 approached the national secondary maximum contaminant level. Coal burning contributed significantly to PM2.5, PM10 and SO2 pollution, especially in winter34. NO2 is mainly emitted into the air by burning fossil fuels, especially vehicle fuels. O3 is formed by the interaction of nitrogen oxides (NOx) and volatile organic compounds (VOCs) with sunlight and it is also high positive correlation with vehicles exhaust emissions35. Using gasoline or diesel fuel cars and using carbon compounds in industrial processes are the major two responsibilities for CO being discharged to the atmosphere36,37.
PM10 and PM2.5 are mainly deposited on the trachea, especially PM2.5 has a huge surface area and adsorbs various toxic and harmful substances, and can enter into the terminal bronchioles and alveoli38, leading to a series of lung injuries including destruction of the airway epithelial barrier, interfered cellular signaling pathways, destroyed lung parenchyma, cell immunity, epigenetic modifications and autophagy39. Reports demonstrated that exposed to high-level SO2 had irritative effects on the smooth muscle of the respiratory tract, causing bronchoconstriction and inflammation of the upper airway, increase airway resistance, and decrease lung function40,41. NO2 is also regarded as a kind of respiratory tract irritation. Animal and human exposure studies have found that inhalation of NO2 mainly invaded in the distal bronchioalveolar and the alveoli, causing respiratory tract infections and promoting lung inflammation42. O3, as a strong corrosive substance, can cause adverse effects on the respiratory system through inflammation, oxidative stress, airway hyperresponsiveness, DNA damage, and another mechanism causing and aggravating pulmonary disease43. Acute and long-term exposure to high-level CO in enclosed spaces can cause serious health hazards to humans, including death, while number of studies found that associations between low-level CO and beneficial in pulmonary health44,45.
The influences of air pollutants on health are complex and vary with different genders, ages, and other risk factors.
From the gender perspective, most studies have discovered that females were more vulnerable to pollutants than males. In this study,
We also found that calculating the overall cumulative risks of PM10, SO2, and NO2 concentrations increasing by 10µg/m3, lagging 15 days, females were significantly higher than males. It indicated that female was more susceptible and threatened potentially by pollutants. However, some studies suggested contrary view, in which males were more susceptible to particulate matter (PM2.5, PM10) exposures than females29,46. There were also studies that showed that in Arak city of Iran, PM2.5, PM10, SO2, NO2 and CO had significantly higher influence on the hospitalization of males with respiratory diseases than that of females, and there was no gender difference in the influence on the hospitalization of circulatory disease47,48. The differences in lung growth rates and function decrease between men and women may influence the incidence of respiratory inflammation49,50. In addition, particulate matter was deposited in women's lungs more than in men, which making them more susceptible to respiratory disease42,51. Complexity was reflected in the actual exposure environment, occupation characteristics, smoking behavior, education level and ethnicity of different gender population, which impacted the health effects of air pollutant exposure52. Such as both active and passive smoking can lead to a decline in lung function, especially in small airway, moreover, the decline in lung function of active smoking people was more serious than passive smoking53. Therefore, the impact of air pollution on gender differences still needs to be further studied.
From the perspective of age, adolescents and elderly groups were more vulnerable to air pollutants than in adults aged 19–59 years old.
In this study, the total cumulative effect of each pollutant increased by 10 µg/m3 was calculated, the incidence of URTI outpatient visits were different in different age groups. For PM10, SO2 and NO2, the risk effects were greater in the adolescents (aged ≤ 18 years old) than in adults (aged 19–59 years old), while for the effect of CO was greater in the adolescents and adults than in the elderly. It indicated that elderly and adolescents were more susceptible to air pollutants than adults, except CO exposure. These results were consistent to previous studies. Li reported that exposed by air pollution in short-term was related to increased risk of URTI for aged 0–14 years old in Hefei14. A study from Beijing indicated that the elder more than 65 years old was the most sensitive to air pollution, followed by adolescents younger than 14 years old54. There are two reasons for the above phenomenon: firstly, compared with adults, adolescents' lungs and immune system are not mature, which were susceptible to air pollutions and adolescents tend to spend more time doing intense outdoors activities, so they may breathe a higher amount of outdoor air pollutants55,56; secondly, for the elderly, both physical function and physical activity had been on a downward trend, showing a decline in the ability of the immune system to fight against exogenous hazards, as well as the cumulative effect caused by long-term exposed from air pollutants, making the elderly more susceptible to air pollutants57. It also implied that the health effects of air pollutant exposure in different age groups were related to the actual exposure level, the sensitivity of pollutant exposure (long-term exposure to pollutants reduces sensitivity to pollutants, people in clean air environment were more sensitive), and the physiological characteristics of aging58,59.
However, there were several limitations in this study: 1) due to the interaction of the pollutants, there was bias to evaluate the risk effects of each single pollutant; 2) the risk effects of indoor pollutants were not controlled; 3) the impacts of individual living habits and occupations on URTI were not considered.