Gaseous Air Pollutants

Gaseous air pollutants are the reactive gases that contaminate the air alongside particulate matter, the most important for health being ground-level ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. They arise largely from combustion and atmospheric chemistry and are associated with respiratory and cardiovascular effects.

Definition

Gaseous air pollutants are harmful gases present in ambient or indoor air, principally ground-level ozone (a secondary pollutant formed photochemically), nitrogen dioxide and sulfur dioxide (largely from combustion), and carbon monoxide (from incomplete combustion).

Scope

This topic covers the principal gaseous pollutants, their sources and atmospheric behaviour, how they injure the airways and circulation, and the epidemiological evidence linking them to mortality and respiratory disease such as asthma. It addresses gases as a pollutant class, complementing the particulate-matter topic, and notes that gaseous and particulate exposures often co-occur.

Key concepts

• Ground-level (tropospheric) ozone
• Nitrogen dioxide and nitrogen oxides
• Sulfur dioxide
• Carbon monoxide
• Primary versus secondary (photochemically formed) pollutants
• Airway oxidative injury and inflammation
• Co-exposure with particulate matter

Mechanisms

Reactive gases act mainly on the respiratory tract: ozone and nitrogen dioxide are oxidant gases that injure airway epithelium and provoke inflammation and airway hyperresponsiveness, sulfur dioxide is a soluble irritant of the upper airways, and carbon monoxide binds haemoglobin and impairs oxygen delivery. Ground-level ozone is not emitted directly but forms through photochemical reactions of precursor pollutants in sunlight, so its concentrations vary with weather and season.

Clinical relevance

Gaseous pollutants are associated at the population level with respiratory outcomes, including asthma exacerbation, and with daily mortality and hospital admissions, informing air-quality surveillance and standard-setting. This entry summarises population-level exposure-outcome evidence and is not a basis for individual diagnosis or treatment.

Epidemiology

Cohort studies link long-term ozone exposure to increased respiratory mortality and long-term nitrogen dioxide to overall mortality, while systematic reviews of time-series data associate short-term nitrogen dioxide with daily mortality and hospital admissions; outdoor gaseous pollutants are also implicated in the development and exacerbation of asthma.

Evidence & guidelines

The ozone-mortality cohort findings of Jerrett and colleagues, the nitrogen dioxide systematic review of Mills and colleagues, and multipollutant cohort analyses such as CanCHEC inform the evidence base, though disentangling gaseous effects from co-occurring particulate matter remains a methodological challenge. The WHO global air quality guidelines (2021) set recommended levels for ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide.

Debates

Can the health effects of gaseous pollutants be separated from particulate matter?

Because gaseous and particulate pollutants share combustion sources and co-vary in time and space, attributing independent effects to gases such as nitrogen dioxide is difficult, and some associations may partly reflect correlated particulate exposure; multipollutant models attempt to address this.

Seminal works

• jerrett-2009
• mills-2015
• guarnieri-2014

Frequently asked questions

Which gaseous air pollutants are most important for health?

Ground-level ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide are the gaseous pollutants of greatest health concern and the ones for which the WHO air quality guidelines set recommended levels.

Why is ozone called a secondary pollutant?

Ground-level ozone is not emitted directly; it forms through photochemical reactions of precursor pollutants, such as nitrogen oxides and volatile organic compounds, in the presence of sunlight.