What is Haze?

The haze we see in our city skyline is caused by tiny particulates suspended in the atmosphere. At high concentrations, these particulates scatter and absorb sunlight resulting in diminished horizontal visibility thereby giving the atmosphere a characteristic opalescent appearance. Haze is not confined to urban environments; it may also be observed in rural areas.

Haze Particles

The particles that cause the haze phenomenon can originate from many sources, some of which are natural and some anthropogenic. Natural sources include the oceans, forests and ground surface. However the majority of the particulates are from human activities which include open burning, land clearing, vehicular use and combustion of fossil fuels in industrial boilers.

Atmospheric Conditions That Favour Haze Formation

In the tropics, wind speeds are generally low. During certain periods of the year, particularly during the Southwest Monsoon season, the atmosphere is very stable and horizontal as well as vertical air movements are reduced. The subsiding air mass does not favour active cloud development, thus the weather is dry. Several factors such as prolonged dry weather, a stable atmosphere, and an abundant supply of pollutants from urban or rural sources are the ideal ingredients for the formation of haze. Particulates emitted into the atmosphere are trapped within the stagnant air mass causing the particulate concentration to increase thus producing a hazy condition. In Peninsular Malaysia, haze is most likely to occur during the months from January to February and June to August.

On the other hand, during the Northeast Monsoon season, the moisture-rich northeasterly winds blowing from the South China Sea are generally stronger and therefore more effective in dispersing haze pollutants. The Northeast Monsoon is often associated with more frequent widespread rain, thus haze is seldom observed.

What Causes The Haze To Clear?

In our changing atmosphere, just as there are hazy days, there are also days which are clear and clean. What can bring about this change? Particulates in the atmosphere are removed through several processes. A proportion, particularly the heavier particles, settles to the ground or onto surfaces by gravitational sedimentation.

The finer particles are removed by a process called “rainout” involving condensation of water vapour onto minute particles to form water droplets eventually producing clouds. Particles are also removed below cloud by collision and adhesion with falling raindrops through another process called “washout”.

Yet another removal mechanism involves the role of turbulence in transporting the haze particulates up to the higher levels of the atmosphere for more effective dispersion. In this case, widespread heavy rain/thunderstorms with strong updrafts and downdrafts are more effective, whereas light rain showers of short duration are usually quite inefficient in cleansing the atmosphere in a severe haze situation.

Haze, Mist or Fog?

Often, haze is confused with mist and fog. During the early morning or after rain showers when temperatures are low and humidity is high, mist and sometimes fog forms in valleys and lowlands, gradually clearing when the sun reappears. Mist and fog are formed as a result of the condensation of water vapour on particulates suspended in the atmosphere. Meteorologically, mist is defined as being present if diminished visibility occurs (with no other weather condition being present) and relative humidity of the atmosphere at the surface of the earth is above 95%. When the horizontal visibility falls below 1000 metres the phenomenon is classified as fog.

Is Haze Harmful?

Haze originating from natural sources such as sea salt and soil dust is generally harmless to humans. Slight, transient haze visible in most towns and cities has not been proven to cause any serious long-term health effects on the population. However, during a severe haze episode, prolonged exposure to high concentrations of particulates can be harmful to health. Associated with previous haze episodes, there have been reports of increases in incidences of eye and throat irritations and respiratory difficulties among sensitive groups. During a haze episode, the public are strongly urged to follow the advice issued by the local health authorities.

How Can We Reduce The Incidence Of Haze?

Firstly, we should refrain from open burning of waste. Most incidences of local haze can be traced to this activity. Motor vehicles, due to their increasing numbers, are also major sources of haze particulates in urban areas. To reduce haze, vehicles should be well maintained. Choose fuels which are the least polluting. Large scale land clearing which exposes large tracts of barren land, not only results in soil erosion and degradation, but also provides a source of particulates to the atmosphere. Promoting efforts such as rapid replanting and staggered land clearing can reduce haze. Remember that every individual has a role to play to preserve the quality of our environment as a legacy for our future generations.

Air Pollutant Index (API)

The ambient air quality measurement in Malaysia is described in terms of Air Pollutant Index (API). The API is developed in easily understood ranges of values as a means of reporting the quality of air instead of using the actual concentration of air pollutants. This index also reflects its effect on human health ranging from good to hazardous and also can be categorized according to the action criteria as stipulated in the National Haze Action Plan. The Malaysian API system closely follows the Pollutant Standard Index (PSI) developed by the United States Environmental Protection Agency (US-EPA)1.

API Status of Malaysia
API Status of Malaysia


How is the API calculated ?

Following the requirement of the Malaysian  Ambient Air Quality Standard  (MAAQS) from the standpoint of human health implications, the pollutants was measured at varying averaging time according to WHO standard2. To determine the API for a given time period, the sub-index values (for all five air pollutants included in the API System) were calculated based on the average concentration calculated. The maximum sub-index of all five pollutants was selected as the API and the specific responsible air pollutants for the API value has to be reported to indicate the relevant health effect category and actions to be taken. The process flow chart for calculating API value at a given time was detailed in figure below.

Malaysia API calculation
Malaysia API calculation

The Malaysian Air Quality Guidelines (MAAGs) which form the basis for calculating the API are presented in Table 1. These guidelines have been derived from available scientific and human health data, and basically represent “safe levels” below which no adverse health effects have been observed. The MAAGs are generally comparable to the corresponding air quality standards recommended by the World Health Organization and other countries.

The averaging time, which varies from 1 to 24 hours for the different air pollutants in MAAGs, represents the period of time over which measurements is monitored and reported for the assessment of human health impacts of specific air pollutants. As such, the air pollution indices are normally monitored and reported for the same averaging times as those employed for the air quality standards/guidelines.

The API values are reported for varying averaging time as follows:

a) PM10 and SO2 on 24-hour running averages.
b) CO on 8-hour running averages.
c) NO2 and O3 on 1-hour running averages.

The API for PM10 reflects specifically levels of particulate matter with a diameter of less than 10 micron pollution and it may not be linked directly to visibility factors, as visibility is often determined by results of semi-quantitative observations over relatively shorter time periods.

The API for PM10 reflects specifically levels of particulate matter with a diameter of less than 10 micron pollution and it may not be linked directly to visibility factors, as visibility is often determined by results of semi-quantitative observations over relatively shorter time periods.

The API value reported for a given time period represents the highest API value among all the sub-APIs calculated during that particular time period. The predominant parameter contributing towards a particular API value is normally indicated alongside the API value. This approach is an effort to promote a uniform and comparable API system. Ideally, all sub-API values exceeding the API 100 threshold limit should also be reported in addition to the predominant API value per se.

To determine the API for a given time period, the sub-index values (sub-API) for all 5 air pollutants included in the API system are first calculated using the above mentioned sub-index functions for the air quality data collected from the Continuous Air Quality Monitoring Stations (CAQMs). The corresponding air quality data are subjected to the necessary quality control processes and quality assurance procedures, prior to the sub-index calculations.

Table 1: Malaysia Air Quality Guidelines, Adopted in Air Pollution Index Calculation.











































Why isn’t PM2.5 included in API calculation as practiced by our neighboring country Singapore?

DOE is in the midst of finalising the new Malaysian Air Quality Guidelines to include the standard limit of PM2.5 in the ambient air which based on World Health Organisation (WHO) 2006 Guidelines. Subsequently, we will need to come up with PM2.5 Air Quality Index System and data integration with the existing system in Environmental Data Centre (EDC) prior to including it in API calculation.


Real-Time Monitoring of Haze Pollution Index

Malaysia Air Pollution Index

Singapore Pollution Standards Index

Indonesia Kualitas Udara Informasi Particulat (PM10)

AQICN.org (Regional Air Quality Index)


Regional Fire Hotspot and Haze Monitoring

ASEAN Specialised Meteorological Centre (ASMC)






Click to access What-to-do-when-API-reach-certain-levels.pdf