Introduction

Air pollution is a major public health hazard, particularly in developing countries [1]. Air pollution is defined as the presence of foreign substances in the air that affect the health and well-being of living beings [2]. As the world progresses, air pollution has become a major problem that has to be faced. This problem is likely to have adverse effects on health, even when pollutant levels are within the standards required by legislation.

There has been considerable interest in recent years in the health effects of exposure to both short-term fluctuations and long-term levels of air pollution, in particular common environmental pollutants including particulate matter (PM), ozone (O3), carbon monoxide (CO), nitrogen dioxide (NO2) and sulphur dioxide (SO2) [3]. The potentially deleterious effect of episodes of high air pollution on health has been suspected for more than 50 years [4].

Since the 1990s, many epidemiological studies have demonstrated associations between air pollution levels and human health in terms of hospital admissions [5, 6]. Air pollution are positively associated with hospital admission for cardiovascular disease [3], respiratory disease [7], and gastrointestinal disease [8].

Further understanding about the association of the air pollution and hospital admission will help the policy maker to understand the seriousness of the effect of air pollution thus helping them in planning and strategizing their health system. It will help them in reorganize their resources so they can anticipate the trend of hospital admission cases that was cause by the air pollution.

The aim of this study was to systematically review the evidence concerning air pollution effects on the risk of hospital admission. We hypothesised that increases in PM, O3, CO, NO2, and SO2 levels would be associated with the increasing trend of hospital admission. We would also like to see the diseases that were affected by the air pollution and the components in the air pollution that cause the hospital admission.

Method

Database and sources

We searched five large databases covering health and medical literature which are Sage, Ovid Medline, Science Direct, Wiley, and ProQuest. Articles retrieved were those that were published from 2010 to 2016. Reference lists of all relevant studies were scanned to identify any further studies, and if these revealed that search terms had been missed, extra terms were added to the main database searches. Conference abstracts and unpublished studies were not included in this review.

Search keywords and terms

Our search of database used the following keywords “hospital admission and air pollution”. All sub-terms were also included and we limited the search to studies of humans, published in English.

Inclusion and exclusion criteria

To examine the hypothesis that ambient air pollutant exposure would be associated with risk of hospitalization, studies of any relevant design were included if they presented original data, and included at least one analysis where hospital admission was the specific outcome, and one or more of the following exposures were investigated: PM, O3, CO, NO2, and SO2. We excluded studies in which the authors did not control for (or stratify by) any potential confounding factors or did not report measures of precision or p values for the analysis of interest.

Procedure

Titles and abstracts were screened for relevance, and full-text versions obtained where appropriate for assessment with reference to the inclusion and exclusion criteria; we were able to obtain full-text papers in all cases where required and it was not necessary to contact specific authors. For each study included, the following information was recorded based on prior beliefs about key aspects of study methodology and in order to summarise study quality: study design, study population, event of interest, number included, location and setting, time period, exposure variables, adjustment for weather variables, and other potential confounders, lags considered. The main results of each study were also recorded – in particular, the effects of each pollutant of interest on risk of hospitalization, including effect sizes and confidence intervals where possible.

Flow diagram of the search study are shown in Figure 1 below.

Figure 1 

Search results and selection of studies for systematic review.

Results

From the 22 articles, nine were time-series (TS) study design, five cohort, five case-crossover (CCO), two retrospective cross-sectional (RCS), and one combination of time series and case-crossover study. Some of the studies focus on the hospital admission of certain group of disease such as cardiovascular and respiratory, while some of them more directly focus on the certain diseases. Table 1 below showed the studies that were involved in this systemic review.

Table 1

Studies that were involved in this systemic review.

No Study Study period Location Study design Health effect

1 Montresor-López et al. 2015 [9] 2002–2006 US Case-crossover Stroke admission
2 Cheng et al. 2015 [10] 2006–2010 Taiwan Case-crossover COPD admission
Asthma admission
Pneumonia admission
3 Milojevic et al. 2014 [11] 2003–2009 England Case-crossover CVS admission
Atrial fibrillation
Arrhythmia
Heart failure
4 Iskandar et al. 2011 [12] 2001–2008 Denmark Case-crossover Asthma admission
5 Ghozikali et al. 2015 [13] 2008–2009 Iran Case-crossover COPD admission
6 Wong et al. 2016 [8] 1998–2001 Hong Kong Cohort Peptic ulcer admission
Gastric ulcer
Duodenal ulcer
7 Tonne et al. 2016 [14] 2003–2007 London Cohort Readmission of Myocardial infarction
8 Atkinson et al. 2014 [15] 2003–2007 London Cohort COPD admission
9 Andersen et al. 2011 [16] 1993–2006 Denmark Cohort COPD admission
10 Andersen et al. 2012 [17] 1993–2007 Denmark Cohort Asthma admission
11 Alimohammadi et al. 2016 [18] 2012–2013 Iran Retrospective cross sectional Ischaemic stroke admission
12 Mansourian et al. 2010 [19] 2005–2006 Iran Retrospective cross sectional Respiratory admission
13 Ferreira et al. 2016 [20] 2010–2011 Brazil Times-series Respiratory admission
CVS Admission
14 Phung et al. 2016 [21] 2004–2007 Vietnam Times-series Respiratory admission
CVS Admission
15 Vidotto et al. 2012 [22] 2000–2007 Brazil Times-series Paediatric rheumatic diseases admission
16 Kollanus et al. 2016 [23] 2001–2010 Finland Times-series CVS admission
Respiratory admission
17 Oudin et al. 2010 [24] 2001–2005 Sweden Times-series Ischaemic stroke admission
18 Vidale et al. 2010 [25] 2000–2003 Italy Times-series Ischaemic stroke admission
19 Jevtić et al. 2014 [26] 2007–2009 Serbia Times-series CVS admission
20 Xie et al. 2014 [27] 2010–2012 China Times-series IHD admission
21 Zhang et al. 2014 [28] 2008–2011 China Times-series Hospital admission
Respiratory admission
Stroke admission
22 Chen et al. 2016 [29] 2003–2013 Adelaide Times-series & case-crossover Asthma admission

Note: CVS: Cardiovascular; IHD: Ischaemic Heart Disease; COPD: Chronic Obstructive Pulmonary Disease.

The trend of hospitalization

The air pollution has cause the increasing trend of hospitalization. Air pollution was noted to have an excessive risk of 3.46 (95% CI, 1.67, 5.27) of total hospital admissions [28]. Most other studies shown the increasing trend of hospitalization by the disease group such as cardiovascular admission and respiratory admission.

Cardiovascular admission was noted to have an increased risk of hospitalization between 1.5 to 2.0 for PM2.5 [20, 23]; (1.007 to 2.7) for PM10 [20, 21]; (1.04 to 1.17) for NO2 [11, 21, 26] and 1.007 for SO2 [21].

For respiratory admission, PM2.5 can caused an increased risk of hospitalization by 1.1 to 1.8 [20, 23]; (1.007 to 1.13) for PM10 [20, 21]; (1.08 to 1.94) for NO2 [21, 28] and 1.02 for SO2 [21].

The rest of the studies showed the effect of the air pollutant to certain disease such as asthma, pneumonia, stroke, and others are shown in Table 2.

Table 2

Effect of the pollutant to hospitalization.

Pollutant Health effect RR/OR/HR (95% CI) Study design Study

PM2.5 Respiratory admission RR 8.5% (–6.8, 26.3) TS Ferreira et al. 2016
RR 10.5% (–2.2, 24.8) TS Kollanus et al. 2016
Asthma admission RR 30.2% (13.4, 49.6)
OR 1.229 (1.139, 1.327)
TS & CCO Chen et al. 2016
OR 1.09 (1.04, 1.13) CCO Iskandar et al. 2011
OR 1.10 (1.06, 1.13) CCO Cheng et al. 2015
COPD Admission OR 1.11 (1.09, 1.13) CCO Cheng et al. 2015
HR 1.05 (0.98, 1.13) Cohort Atkinson et al. 2014
Pneumonia admission OR 1.12 (1.11, 1.13) CCO Cheng et al. 2015
CVS Admission RR 19.6% (6.4, 34.6) TS Ferreira et al. 2016
RR 1.5% (–6.9, 10.6) TS Kollanus et al. 2016
Ischaemic stroke RR 1.09 (1.03, 1.15) RCS Alimohammadi et al. 2016
IHD admission RR 0.27% (0.21, 0.33) TS Xie et al. 2014
MI admission HR 1.02 (0.98, 1.06) Cohort Tonne et al. 2016
PUD admission HR 1.18 (1.02, 1.36) Cohort Wong et al. 2016
Gastric ulcer HR 1.29 (1.09, 1.53)
Duodenal ulcer HR 0.98 (0.78, 1.22)
PM10 Respiratory admission RR 12.8% (6.0, 20.0) TS Ferreira et al. 2016
β coefficient = 0.63; p < 0.001) RCS Mansourian et al. 2010
RR 1.007 (1.002, 1.013) TS Phung et al. 2016
Asthma admission RR 8.3% (2.5, 14.4)
OR 1.035 (1.007, 1.064)
TS & CCO Chen et al. 2016
OR 1.04 (1.03–1.06) CCO Cheng et al. 2015
OR 1.07 (1.03, 1.12) CCO Iskandar et al. 2011
COPD Admission OR 1.05 (1.03–1.06) CCO Cheng et al. 2015
Pneumonia admission OR 1.05 (1.04–1.05) CCO Cheng et al. 2015
CVS Admission RR 2.7% (–2.2, 7.9) TS Ferreira et al. 2016
RR 1.005 (1, 1.009) TS Phung et al. 2016
Ischaemic stroke RR 1.14 (1.06, 1.22) RCS Alimohammadi et al. 2016
RR 13% (4, 22) TS Oudin et al. 2010
RR 1.078 (1.104, 1.052) TS Vidale et al. 2010
MI admission HR 1.05 (1.00, 1.10) Cohort Tonne et al. 2016
NO2 Respiratory admission RR 1.08 (1.06, 1.011) TS Phung et al. 2016
RR 1.94 (0.50, 3.40) TS Zhang et al. 2014
Asthma admission RR 12.5% (6.6, 18.7),
OR 1.077 (1.046, 1.109)
TS & CCO Chen et al. 2016
HR 1.12 (1.04, 1.22) Cohort Andersen et al. 2012
OR 1.10 (1.04, 1.16) CCO Iskandar et al. 2011
COPD Admission HR 1.06 (0.98, 1.15) Cohort Atkinson et al. 2014
HR 1.08 (1.02, 1.14) Cohort Andersen et al. 2011
OR 1.0038 (1.0004, 1.0094) CCO Ghozikali et al. 2015
CVS Admission RR 1.04 (1, 1.06) TS Phung et al. 2016
RR 1.049 (1.009, 1.091) TS Jevtić et al. 2014
OR 1.7% (95% CI 0.9 to 2.6) CCO Milojevic et al. 2014
Ischaemic stroke RR 1.07 (1.04, 1.1) RCS Alimohammadi et al. 2016
RR 1.039 (1.066, 1.013) TS Vidale et al. 2010
MI admission HR 1.05 (0.99, 1.10) Cohort Tonne et al. 2016
SO2 Respiratory admission RR 1.02 (1.01, 1.03) TS Phung et al. 2016
β coefficient = 0.59; p < 0.001) RCS Mansourian et al. 2010
COPD admission OR 1.0044 (1, 1.011) CCO Ghozikali et al. 2015
CVS Admission RR 1.007 (1, 1.01) TS Phung et al. 2016
Ischaemic stroke RR 1.08 (1.06, 1.1) RCS Alimohammadi et al. 2016
Paediatric rheumatic diseases RR 1.98% (0.25, 3.69) TS Vidotto et al. 2012
Ozone COPD admission RR 1.0058 (1.0022, 1.0094) CCO Ghozikali et al. 2015
Ischaemic stroke RR 1.07 (1.03, 1.11) RCS Alimohammadi et al. 2016
Stroke admission OR 0.98 (0.96, 1.00) CCO Montresor-López et al. 2015

Note: CVS: Cardiovascular; IHD: Ischaemic Heart Disease; COPD: Chronic Obstructive Pulmonary Disease; MI: Myocardial infarction; PUD: Peptic Ulcer Disease.

Type of AP that cause the hospitalization

In this review, five air pollutant were identified that cause their own health effect. They are particulate matter less than 2.5 μm (PM2.5), particulate matter less than 10 μm (PM10), nitrogen dioxide (NO2), sulphur dioxide (SO2), and ozone. Each pollutant has their own effect to certain disease as shown by the relative risk (RR), odds ratio (OR), and hazard ratio (HR) depending on the type of the study design.

For example, PM2.5 has effect on asthma, pneumonia, COPD, stroke, Ischaemic heart Disease (IHD), myocardial infarct (MI), gastric ulcer, and other admission. Table 2 summarizes the effect of these pollutants.

Diseases that were affected by air pollution

From this review, several diseases were identified that has association with the air pollutants. For certain disease, such as asthma, PM2.5, PM10, and NO2 contribute to the hospitalization with certain effect size as shown by the RR, OR, or HR depending the type of the study design. The rest of the diseases with the certain type of pollutants that effect the admission are shown in Table 3 below.

Table 3

The disease and the pollutant that effect the admission.

Diseases Pollutant Study Study design Effect

Asthma PM2.5 Chen et al. 2016 TS & CCO RR 30.2% (13.4, 49.6)
OR 1.229 (1.139, 1.327)
Cheng et al. 2015 CCO OR 1.10 (1.06, 1.13)
Iskandar et al. 2011 CCO OR 1.09 (1.04, 1.13)
PM10 Chen et al. 2016 TS & CCO RR 8.3% (2.5, 14.4)
OR 1.035 (1.007, 1.064)
Cheng et al. 2015 CCO OR 1.04 (1.03–1.06)
Iskandar et al. 2011 CCO OR 1.07 (1.03, 1.12)
NO2 Chen et al. 2016 TS & CCO RR 12.5% (6.6, 18.7),
OR 1.077 (1.046, 1.109)
Iskandar et al. 2011 CCO OR 1.10 (1.04, 1.16)
Andersen et al. 2012 Cohort HR 1.12 (1.04, 1.22)
COPD PM2.5 Cheng et al. 2015 CCO OR 1.11 (1.09, 1.13)
Atkinson et al. 2014 Cohort HR 1.05 (0.98, 1.13)
PM10 Cheng et al. 2015 CCO OR 1.05 (1.03–1.06)
NO2 Atkinson et al. 2014 Cohort HR 1.06 (0.98, 1.15)
Andersen et al. 2011 Cohort HR 1.08 (1.02, 1.14)
Ghozikali et al. 2015 CCO RR 1.0038 (1.0004, 1.0094)
SO2 Ghozikali et al. 2015 CCO RR 1.0044 (1, 1.011)
Ozone Ghozikali et al. 2015 CCO RR 1.0058 (1.0022, 1.0094)
Pneumonia PM2.5 Cheng et al. 2015 CCO OR 1.12 (1.11, 1.13)
PM10 Cheng et al. 2015 CCO OR 1.05 (1.03–1.06)
Ischaemic stroke PM2.5 Hossein RCS RR 1.09 (1.03, 1.15)
PM10 Alimohammadi et al. 2016 RCS RR 1.14 (1.06, 1.22)
Oudin et al. 2010 TS RR 13% (4, 22)
Vidale et al. 2010 TS RR 1.078 (1.104, 1.052)
NO2 Alimohammadi et al. 2016 RCS RR 1.07 (1.04, 1.1)
Vidale et al. 2010 TS RR 1.039 (1.066, 1.013)
SO2 Alimohammadi et al. 2016 RCS RR 1.08 (1.06, 1.1)
Ozone Alimohammadi et al. 2016 RCS RR 1.07 (1.03, 1.11)
Montresor-López et al. 2015 CCO OR 0.98 (0.96, 1.00)

Discussion

Main results and comparisons with existing literature

Our systematic review of literature of 22 studies on the effect of air pollution and hospital admission showed that there are increasing risk of hospital admission for cardiovascular and respiratory group of diseases. Air pollution was believed to have influence only the respiratory disease such as asthma and COPD in the old studies in the 1990. However, in the early 2000, more studies were done to establish the connection of the air pollution with the cardiovascular disease as we have more understanding of the components in the air pollutant and the physiology that they can cause to the human body. Therefore, our study concurrent with the other study that state that air pollution cause higher risk of cardiovascular and respiratory disease hospitalization [30, 31, 32, 33, 34].

This paper also showed that the PM either the fine particulate (PM2.5) or coarse particulate (PM10) has a higher influence of hospital admission either in cardiovascular or respiratory disease than the other air pollutants. This was due to the fact that physiology of the PM that can penetrate deep into the lungs and heart and alters the autonomic control of the heart which lead to cardiovascular problem [35]. It also act as an irritant and induce defensive responses in the airways, such as increased mucus secretion and increased bronchial hyperactivity and lead to respiratory problem [36]. This finding is congruent with other studies as well stated that increased concentration of PM associated with hospitalization [30, 31, 32].

Weaknesses of our methodology

Despite carrying out a comprehensive search some studies may have been missed to be included for this systematic review. However, by searching a number of different databases, with different indexing systems, and, furthermore, checking reference lists and the websites of major organisations, we believe that all major studies with hospital admission as the primary outcome have been picked up. In addition, there might have been publication bias: studies finding effects may have been more likely to be published. The extent of publication bias is difficult to assess in studies with such varied methodology and reporting. Though such concerns should always be borne in mind, our goal was not to produce a definitive numerical estimate of the effects of air pollution on hospital admission risk, but rather to give an overview of the evidence available. Finally, since we excluded non-English abstracts citations owing to resource limitations and may have missed some non-English full-text articles, we believe that this is unlikely to have led to the omission of any major papers in the area.

Impact of our results

The result of this paper strongly supports the fact that the effect of air pollution is associated with the higher risk of hospital admission for cardiovascular and respiratory diseases. This is supported by several systemic reviews done previously [3, 32, 37]. It is plausible that morbidity and mortality from non-communicable diseases such as stroke and ischaemic heart disease, the impact of air pollution is also an important and act as modifiable risk factor [30, 31]. Understanding this, it should give enough evidence to the policy makers to make same action and plan to reduce this effect.

Future research

The exact role of individual pollutants is still unclear, and perhaps only further experimental studies under controlled conditions can deal with this issue. There is also a need for biomarkers of exposure that can be used in epidemiological studies to give more reliable estimates of individual exposure to air pollutants. There is also a need for more studies that take into account the potential effect modifiers; though a few studies have presented stratified or age-restricted data, there is little direct evidence on how age, and other individual-level factors such as previous disease, affect a person’s vulnerability. Finally, future studies are needed to ascertain factors contributing to why some people or indeed populations are more susceptible than others to the detrimental effects of air pollution.

Conclusion

The exposure to air pollutants confers to an increased risk of hospital admission of several disease. Our findings call for greater awareness of environmental protection and the implementation of effective measures to improve the quality of air, which may reduce the risks of adverse effects on the population’s health. Public and environmental health policies that aim to reduce air pollution levels might reduce the burden of multiple diseases such as stroke, asthma, and ischaemic heart diseases that are influenced by the air pollutants.