The Malaysian Air Pollution Index (API) and the US Air Quality Index (AQI), which AirVisual uses by default, are as follows: 

The Malaysian Air Pollution Index (left) and the U.S. Air Quality Index (right) which AirVisual uses by default
In August 2018, Malaysia added PM2.5 to the calculation of its Air Pollution Index, which means it now bases its Air Pollution Index on the average concentrations of six air pollutants: PM2.5, PM10, PM2.5, sulphur dioxide, nitrogen dioxide, carbon monoxide and ozone.

The Malaysian Air Pollution Index uses a moving 24-hour average for PM2.5, PM10 and SO2; an 8-hour average for CO and O3; and a 1-hour average for NOx).
You can read more about their API on their government website here

(The US AQI uses a 24-hour average for PM2.5, but AirVisual uses the hourly concentration to calculate AQI, because we want to give real-time information to people so they can take actions, where necessary, to protect their health.)

The US AQI and the Malaysian API both use the same six pollutants, and both use the dominant pollutant – the pollutant with the highest Index value – to determine the overall Index value. In Malaysia, PM2.5 is usually the highest and therefore determines the overall Air Quality Index or Air Pollution Index value most of the time. 

What may be different with the US AQI is the PM2.5 concentrations that are used to determine whether air quality falls into the “good” category and upwards. There is a slight difference with PM10: for example, for a value between 0-50 (“Good”) on the US Air Quality Index or Malaysian Air Pollution Index, the PM10 concentrations can go up to 54 µg/m³ on the US scale and up to 50 µg/m³ on the Malaysian scale. However, the biggest differences between the US AQI and those of other countries are usually seen in the PM2.5 concentration scale. The Malaysian Department of Environment’s PM2.5 concentration scale is not available online, so currently we do not know how they calculate the PM2.5 Air Pollution Index value.