title: Quantitative systematic review of the associations between short-term exposure to nitrogen dioxide and mortality and hospital admissions.
abstract: BACKGROUND
  Short-term exposure to NO₂ has been associated with adverse health effects and there is increasing concern that NO₂ is causally related to health effects, not merely a marker of traffic-generated pollution. No comprehensive meta-analysis of the time-series evidence on NO₂ has been published since 2007.
  
  
  OBJECTIVE
  To quantitatively assess the evidence from epidemiological time-series studies published worldwide to determine whether and to what extent short-term exposure to NO₂ is associated with increased numbers of daily deaths and hospital admissions.
  
  
  METHODS
  We conducted a quantitative systematic review of 204 time-series studies of NO₂ and daily mortality and hospital admissions for several diagnoses and ages, which were indexed in three bibliographic databases up to May 2011. We calculated random-effects estimates by different geographic regions and globally, and also tested for heterogeneity and small study bias.
  
  
  RESULTS
  Sufficient estimates for meta-analysis were available for 43 cause-specific and age-specific combinations of mortality or hospital admissions (25 for 24 h NO₂ and 18 of the same combinations for 1 h measures). For the all-age group, a 10 µg/m(3) increase in 24 h NO₂ was associated with increases in all-cause, cardiovascular and respiratory mortality (0.71% (95% CI 0.43% to 1.00%), 0.88% (0.63% to 1.13%) and 1.09% (0.75% to 1.42%), respectively), and with hospital admissions for respiratory (0.57% (0.33% to 0.82%)) and cardiovascular (0.66% (0.32% to 1.01%)) diseases. Evidence of heterogeneity between geographical region-specific estimates was identified in more than half of the combinations analysed.
  
  
  CONCLUSIONS
  Our review provides clear evidence of health effects associated with short-term exposure to NO₂ although further work is required to understand reasons for the regional heterogeneity observed. The growing literature, incorporating large multicentre studies and new evidence from less well-studied regions of the world, supports further quantitative review to assess the independence of NO₂ health effects from other air pollutants.
authors.affiliation: Public Health England, Centre for Radiation, Chemical and Environmental Hazards, Oxfordshire, UK.
authors.forename: I C
authors.initials: IC
authors.lastname: Mills
authors.affiliation: Population Health Research Institute and MRC-PHE Centre for Environment and Health, St George's, University of London, London, UK.
authors.forename: R W
authors.initials: RW
authors.lastname: Atkinson
authors.affiliation: Population Health Research Institute and MRC-PHE Centre for Environment and Health, St George's, University of London, London, UK.
authors.forename: S
authors.initials: S
authors.lastname: Kang
authors.affiliation: MRC-PHE Centre for Environment and Health, King's College London, London, UK NIHR Biomedical Research Centre at Guy's and St Thomas' NHS Foundation Trust and King's College London, London, UK NIHR Health Protection Research Unit on Environmental Hazards at King's College London in partnership with Public Health England, London, UK.
authors.forename: H
authors.identifier: http://orcid.org/0000-0002-9252-3962
authors.initials: H
authors.lastname: Walton
authors.affiliation: Population Health Research Institute and MRC-PHE Centre for Environment and Health, St George's, University of London, London, UK MRC-PHE Centre for Environment and Health, King's College London, London, UK.
authors.forename: H R
authors.initials: HR
authors.lastname: Anderson
mesh_terms: D000393:Air Pollutants / Q000009:adverse effects* / Q000032:analysis; D000397:Air Pollution / Q000009:adverse effects / Q000517:prevention & control*; D002318:Cardiovascular Diseases / Q000453:epidemiology / Q000209:etiology / Q000401:mortality*; D004784:Environmental Monitoring; D005069:Evaluation Studies as Topic; D006760:Hospitalization / Q000706:statistics & numerical data*; D006801:Humans; D008171:Lung Diseases / Q000453:epidemiology / Q000209:etiology / Q000401:mortality*; D015337:Multicenter Studies as Topic; D009585:Nitrogen Dioxide / Q000009:adverse effects* / Q000032:analysis; D052638:Particulate Matter; D013997:Time Factors
chemical_list: D000393:Air Pollutants; D052638:Particulate Matter; D009585:Nitrogen Dioxide
keywords: EPIDEMIOLOGY; PUBLIC HEALTH