Introduction

Air pollution is a major public health hazard, particularly in developing countries []. Air pollution is defined as the presence of foreign substances in the air that affect the health and well-being of living beings []. 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) []. The potentially deleterious effect of episodes of high air pollution on health has been suspected for more than 50 years [].

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

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 Conference abstracts and unpublished studies were not included in this review.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.

NoStudyStudy periodLocationStudy designHealth effect

1Montresor-López et al. 2015 []2002–2006USCase-crossoverStroke admission
2Cheng et al. 2015 []2006–2010TaiwanCase-crossoverCOPD admission
Asthma admission
Pneumonia admission
3Milojevic et al. 2014 []2003–2009EnglandCase-crossoverCVS admission
Atrial fibrillation
Arrhythmia
Heart failure
4Iskandar et al. 2011 []2001–2008DenmarkCase-crossoverAsthma admission
5Ghozikali et al. 2015 []2008–2009IranCase-crossoverCOPD admission
6Wong et al. 2016 []1998–2001Hong KongCohortPeptic ulcer admission
Gastric ulcer
Duodenal ulcer
7Tonne et al. 2016 []2003–2007LondonCohortReadmission of Myocardial infarction
8Atkinson et al. 2014 []2003–2007LondonCohortCOPD admission
9Andersen et al. 2011 []1993–2006DenmarkCohortCOPD admission
10Andersen et al. 2012 []1993–2007DenmarkCohortAsthma admission
11Alimohammadi et al. 2016 []2012–2013IranRetrospective cross sectionalIschaemic stroke admission
12Mansourian et al. 2010 []2005–2006IranRetrospective cross sectionalRespiratory admission
13Ferreira et al. 2016 []2010–2011BrazilTimes-seriesRespiratory admission
CVS Admission
14Phung et al. 2016 []2004–2007VietnamTimes-seriesRespiratory admission
CVS Admission
15Vidotto et al. 2012 []2000–2007BrazilTimes-seriesPaediatric rheumatic diseases admission
16Kollanus et al. 2016 []2001–2010FinlandTimes-seriesCVS admission
Respiratory admission
17Oudin et al. 2010 []2001–2005SwedenTimes-seriesIschaemic stroke admission
18Vidale et al. 2010 []2000–2003ItalyTimes-seriesIschaemic stroke admission
19Jevtić et al. 2014 []2007–2009SerbiaTimes-seriesCVS admission
20Xie et al. 2014 []2010–2012ChinaTimes-seriesIHD admission
21Zhang et al. 2014 []2008–2011ChinaTimes-seriesHospital admission
Respiratory admission
Stroke admission
22Chen et al. 2016 []2003–2013AdelaideTimes-series & case-crossoverAsthma 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 []. 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 [, ]; (1.007 to 2.7) for PM10 [, ]; (1.04 to 1.17) for NO2 [, , ] and 1.007 for SO2 [].

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

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.

PollutantHealth effectRR/OR/HR (95% CI)Study designStudy

PM2.5 Respiratory admissionRR 8.5% (–6.8, 26.3)TSFerreira et al. 2016
RR 10.5% (–2.2, 24.8)TSKollanus et al. 2016
Asthma admissionRR 30.2% (13.4, 49.6)
OR 1.229 (1.139, 1.327)
TS & CCOChen et al. 2016
OR 1.09 (1.04, 1.13)CCOIskandar et al. 2011
OR 1.10 (1.06, 1.13)CCOCheng et al. 2015
COPD AdmissionOR 1.11 (1.09, 1.13)CCOCheng et al. 2015
HR 1.05 (0.98, 1.13)CohortAtkinson et al. 2014
Pneumonia admissionOR 1.12 (1.11, 1.13)CCOCheng et al. 2015
CVS AdmissionRR 19.6% (6.4, 34.6)TSFerreira et al. 2016
RR 1.5% (–6.9, 10.6)TSKollanus et al. 2016
Ischaemic strokeRR 1.09 (1.03, 1.15)RCSAlimohammadi et al. 2016
IHD admissionRR 0.27% (0.21, 0.33)TSXie et al. 2014
MI admissionHR 1.02 (0.98, 1.06)CohortTonne et al. 2016
PUD admissionHR 1.18 (1.02, 1.36)CohortWong et al. 2016
Gastric ulcerHR 1.29 (1.09, 1.53)
Duodenal ulcerHR 0.98 (0.78, 1.22)
PM10 Respiratory admissionRR 12.8% (6.0, 20.0)TSFerreira et al. 2016
β coefficient = 0.63; p < 0.001)RCSMansourian et al. 2010
RR 1.007 (1.002, 1.013)TSPhung et al. 2016
Asthma admissionRR 8.3% (2.5, 14.4)
OR 1.035 (1.007, 1.064)
TS & CCOChen et al. 2016
OR 1.04 (1.03–1.06)CCOCheng et al. 2015
OR 1.07 (1.03, 1.12)CCOIskandar et al. 2011
COPD AdmissionOR 1.05 (1.03–1.06)CCOCheng et al. 2015
Pneumonia admissionOR 1.05 (1.04–1.05)CCOCheng et al. 2015
CVS AdmissionRR 2.7% (–2.2, 7.9)TSFerreira et al. 2016
RR 1.005 (1, 1.009)TSPhung et al. 2016
Ischaemic strokeRR 1.14 (1.06, 1.22)RCSAlimohammadi et al. 2016
RR 13% (4, 22)TSOudin et al. 2010
RR 1.078 (1.104, 1.052)TSVidale et al. 2010
MI admissionHR 1.05 (1.00, 1.10)CohortTonne et al. 2016
NO2 Respiratory admissionRR 1.08 (1.06, 1.011)TSPhung et al. 2016
RR 1.94 (0.50, 3.40)TSZhang et al. 2014
Asthma admissionRR 12.5% (6.6, 18.7),
OR 1.077 (1.046, 1.109)
TS & CCOChen et al. 2016
HR 1.12 (1.04, 1.22)CohortAndersen et al. 2012
OR 1.10 (1.04, 1.16)CCOIskandar et al. 2011
COPD AdmissionHR 1.06 (0.98, 1.15)CohortAtkinson et al. 2014
HR 1.08 (1.02, 1.14)CohortAndersen et al. 2011
OR 1.0038 (1.0004, 1.0094)CCOGhozikali et al. 2015
CVS AdmissionRR 1.04 (1, 1.06)TSPhung et al. 2016
RR 1.049 (1.009, 1.091)TSJevtić et al. 2014
OR 1.7% (95% CI 0.9 to 2.6)CCOMilojevic et al. 2014
Ischaemic strokeRR 1.07 (1.04, 1.1)RCSAlimohammadi et al. 2016
RR 1.039 (1.066, 1.013)TSVidale et al. 2010
MI admissionHR 1.05 (0.99, 1.10)CohortTonne et al. 2016
SO2 Respiratory admissionRR 1.02 (1.01, 1.03)TSPhung et al. 2016
β coefficient = 0.59; p < 0.001)RCSMansourian et al. 2010
COPD admissionOR 1.0044 (1, 1.011)CCOGhozikali et al. 2015
CVS AdmissionRR 1.007 (1, 1.01)TSPhung et al. 2016
Ischaemic strokeRR 1.08 (1.06, 1.1)RCSAlimohammadi et al. 2016
Paediatric rheumatic diseasesRR 1.98% (0.25, 3.69)TSVidotto et al. 2012
OzoneCOPD admissionRR 1.0058 (1.0022, 1.0094)CCOGhozikali et al. 2015
Ischaemic strokeRR 1.07 (1.03, 1.11)RCSAlimohammadi et al. 2016
Stroke admissionOR 0.98 (0.96, 1.00)CCOMontresor-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.

DiseasesPollutantStudyStudy designEffect

AsthmaPM2.5 Chen et al. 2016TS & CCORR 30.2% (13.4, 49.6)
OR 1.229 (1.139, 1.327)
Cheng et al. 2015CCOOR 1.10 (1.06, 1.13)
Iskandar et al. 2011CCOOR 1.09 (1.04, 1.13)
PM10 Chen et al. 2016TS & CCORR 8.3% (2.5, 14.4)
OR 1.035 (1.007, 1.064)
Cheng et al. 2015CCOOR 1.04 (1.03–1.06)
Iskandar et al. 2011CCOOR 1.07 (1.03, 1.12)
NO2 Chen et al. 2016TS & CCORR 12.5% (6.6, 18.7),
OR 1.077 (1.046, 1.109)
Iskandar et al. 2011CCOOR 1.10 (1.04, 1.16)
Andersen et al. 2012CohortHR 1.12 (1.04, 1.22)
COPDPM2.5 Cheng et al. 2015CCOOR 1.11 (1.09, 1.13)
Atkinson et al. 2014CohortHR 1.05 (0.98, 1.13)
PM10 Cheng et al. 2015CCOOR 1.05 (1.03–1.06)
NO2 Atkinson et al. 2014CohortHR 1.06 (0.98, 1.15)
Andersen et al. 2011CohortHR 1.08 (1.02, 1.14)
Ghozikali et al. 2015CCORR 1.0038 (1.0004, 1.0094)
SO2 Ghozikali et al. 2015CCORR 1.0044 (1, 1.011)
OzoneGhozikali et al. 2015CCORR 1.0058 (1.0022, 1.0094)
PneumoniaPM2.5 Cheng et al. 2015CCOOR 1.12 (1.11, 1.13)
PM10 Cheng et al. 2015CCOOR 1.05 (1.03–1.06)
Ischaemic strokePM2.5 HosseinRCSRR 1.09 (1.03, 1.15)
PM10 Alimohammadi et al. 2016RCSRR 1.14 (1.06, 1.22)
Oudin et al. 2010TSRR 13% (4, 22)
Vidale et al. 2010TSRR 1.078 (1.104, 1.052)
NO2 Alimohammadi et al. 2016RCSRR 1.07 (1.04, 1.1)
Vidale et al. 2010TSRR 1.039 (1.066, 1.013)
SO2 Alimohammadi et al. 2016RCSRR 1.08 (1.06, 1.1)
OzoneAlimohammadi et al. 2016RCSRR 1.07 (1.03, 1.11)
Montresor-López et al. 2015CCOOR 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 [, , , , ].

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 []. 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 []. This finding is congruent with other studies as well stated that increased concentration of PM associated with hospitalization [, , ].

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 [, , ]. 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 [, ]. 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.