Haze
Haze is a suspension in the air of extremely small, dry particles invisible to the naked eye and sufficiently numerous to give the air an opalescent appearance (WMO, 2017).
Primary reference(s)
WMO, 2017. Haze. International Cloud Atlas, World Meteorological Organization (WMO). Accessed 16 May 2025.
Annotations
Additional scientific description
As light is scattered by haze particles, distant bright objects or lights seen through the haze appear yellowish or reddish, while dark objects appear bluish. Haze particles may have a colour of their own that also contributes to this effect (WMO, 2017). In addition, haze has been described as particles suspended in air, reducing visibility by scattering light; often a mixture of aerosols and photochemical smog (AMS, 2012).
Some specific types of haze include:
- Dust haze: A suspension in the air of dust or small sand particles, raised from the ground prior to the time of observation by a dust storm or sandstorm. The dust storm or sandstorm may have occurred either at or near the observation site or far from it (WMO, 2017).
- Sand haze: Haze caused by the suspension in the atmosphere of small sand or dust particles, raised from the ground prior to the time of observation by a sandstorm or dust storm (WMO, 1992).
Many aerosols increase in size with increasing relative humidity due to deliquescence, drastically decreasing visibility. On Köhler curve plots of saturation relative humidity versus aerosol particle radius, equilibrium haze particles are to the left of the peak, while growing cloud droplets are to the right (AMS, 2012).
Many haze formations are caused by the presence of an abundance of condensation nuclei, which may grow in size, due to a variety of causes, and become mist, fog, or cloud. Distinction is sometimes drawn between dry haze and damp haze, largely on the basis of differences in optical effects produced by the smaller particles (dry haze) and larger particles (damp haze), which develop from slow condensation upon the hygroscopic haze particles (AMS, 2012).
Dry haze particles, with diameters of the order of 0.1 micron, are small enough to scatter shorter wavelengths of light preferentially (although not according to the inverse fourth-power law of Rayleigh scattering). Such haze particles produce a bluish colour when the haze is viewed against a dark background, for dispersion allows only the slightly bluish scattered light to reach the eye. The same type of haze, when viewed against a light background, appears as a yellowish veil, for here the principal effect is the removal of the bluer components from the light originating in the distant light-coloured background (AMS, 2012).
Haze may be distinguished by this same effect from mist, which yields only a grey obscuration, since the particle sizes are too large to yield appreciable differential scattering of various wavelengths (AMS, 2012).
Metrics and numeric limits
Haze is an atmospheric phenomenon characterized by the accumulation of fine particulate matter, particularly PM₂.₅ (particles with diameters less than 2.5 micrometers), which can severely reduce visibility and degrade air quality. The World Health Organization (WHO) classifies PM₂.₅ as the principal pollutant used to evaluate haze-related health risks, and in its 2021 air quality guidelines, recommends that average annual PM₂.₅ concentrations not exceed 5 µg/m³, with a 24-hour mean limit of 15 µg/m³ to protect public health (WHO, 2021). However, during severe haze episodes, such as those driven by transboundary biomass burning in Southeast Asia, PM₂.₅ concentrations can spike to levels many times higher than these guidelines, posing serious health threats (Cheong et al., 2019). Many countries use the Air Quality Index (AQI) to communicate these risks to the public, with PM₂.₅ concentrations in the "very unhealthy" or "hazardous" range prompting emergency public health advisories and school or workplace closures (U.S. EPA, 2023). These internationally recognized metrics are critical for monitoring air quality, issuing health warnings, and guiding coordinated policy responses to mitigate the impacts of haze.
Key relevant UN convention / multilateral treaty
Sendai Framework for Disaster Risk Reduction 2015-2030.
Drivers
In meteorology, haze is caused by suspended dry solid particles in the air, such as dust or salt. Haze is an atmospheric phenomenon in meteorological observation and the particles of which it is formed are termed lithometeors (AMS, 2012).
Impacts
It can obscure vision and directly influence horizontal visibility (AMS, 2012).
Exposure to haze, particularly from transboundary pollution events in Southeast Asia, has been shown to cause various adverse health effects, with risks escalating significantly with prolonged exposure. Short-term contact with haze can lead to respiratory irritation, reduced lung function, and exacerbation of existing cardiovascular and respiratory conditions. Long-term or repeated exposure, however, has been associated with more severe health outcomes, including increased hospital admissions, heightened mortality risk, and chronic respiratory diseases (Cheong et al., 2019). These findings underscore the critical need for effective public health interventions and regional cooperation to mitigate the health impacts of recurrent haze episodes.
Multi-hazard context
The figure below summarises common interactions between haze and other hazards. This information should be used with caution and not be solely relied upon in Disaster Risk Management, particularly as some interactions may not have been included. Note that hazardous events occurring together or locally in space or time may not necessarily cause, amplify or be otherwise related to each other. Specific examples of multi-hazard context can be found in the ‘Hazard drivers’ and ‘Impacts’ sections above.
Multi-hazard diagram
Risk Management
Risk management for haze involves coordinated strategies across monitoring, early warning, public health protection, and long-term emission reduction. Key measures include the use of air quality monitoring networks to track fine particulate matter (PM₂.₅) and issue timely advisories when concentrations exceed health-based thresholds, such as those recommended by the World Health Organization (WHO, 2021). Governments often implement short-term responses, including school closures, distribution of masks, and public guidance to remain indoors during severe haze events (Cheong et al., 2019). A notable example of haze risk management is Singapore’s transboundary haze action plan, which integrates real-time monitoring, a Pollutant Standards Index (PSI)-based alert system, and cross-border collaboration with ASEAN countries to reduce regional biomass burning (National Environment Agency, 2023). Alerts are conveyed to the public through multiple channels, including mobile apps, SMS notifications, government websites, and daily media broadcasts, enabling individuals and institutions to take timely protective actions. Long-term mitigation involves policies promoting sustainable land management, stricter enforcement against illegal burning, and cleaner energy transitions to reduce the root causes of haze.
Monitoring
The section above and the table below offer an overview of monitoring haze. This information can be used for forecasting within a national early warning system (EWS). Since EWS capacities and processes differ across countries, the most current and specific information regarding EWS should be obtained from the appropriate national or regional agency/authority responsible for disaster management.
| Which institution(s) produce(s) Disaster Risk Data/Information? |
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| How is the Hazard Observed/Monitored/Forecast? | Haze is monitored using ground-based air quality sensors, satellites, and weather radar to track fine particulate matter (PM2.5, PM10) and visibility levels. Meteorologists analysze wind patterns, humidity, and pollution sources such as wildfires and industrial emissions to predict haze formation and movement. Computer models simulate air pollution dispersion, helping forecasters issue warnings. These alerts help protect public health, aviation, and transportation by providing guidance on air quality and visibility conditions. |
References
American Meteorological Society (AMS), 2012. Haze. Glossary of Meteorology. American Meteorological Society (AMS). Accessed 16 May 2025.
Cheong, K.H., Ngiam, N.J., Morgan, G.G., Pek, P.P., Tan, B.Y.-Q., Lai, J.W., Koh, J.M., Ong, M.E.H. and Ho, A.F.W., 2019. ‘Acute health impacts of the Southeast Asian transboundary haze problem—A review’, International Journal of Environmental Research and Public Health, 16(18), p. 3286. Accessed 15 May 2025.
United States Environmental Protection Agency (EPA), 2023. Air Quality Index (AQI) basics. Accessed 16 May 2025.
National Environment Agency (NEA), 2023. Managing haze. Government of Singapore. Accessed 16 May 2025.
World Health Organization (WHO), 2021. WHO global air quality guidelines: Particulate matter (PM₂.₅ and PM₁₀), ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide. Geneva: World Health Organization. Accessed 16 May 2025.
World Meteorological Organization (WMO), 1992. International meteorological vocabulary. WMO-No. 182. World Meteorological Organization. Accessed 16 May 2025.
World Meteorological Organization (WMO), 2017. Haze. International Cloud Atlas, World Meteorological Organization. Accessed 16 May 2025.