The haze will be a recurring problem until next month due to the South-West monsoon.
The monsoon winds, prevalent between April and September, will intermittently carry with it smoke from the Sumatra forest fires.
Occasional showers expected in several parts of the city will only temporarily improve visibility and air quality level.
The visibility level dropped to less than one kilometre at 4pm today in Sepang and Kuala Lumpur International Airport.
Subang, Petaling Jaya and Kuala Lumpur recorded visibility levels of only 4km, while visibility was less than 10km in other parts of the Klang Valley.
The smell of burnt ash permeated the city air yesterday.
A Department of Environment spokesman said the Air Pollutant Index (API) reading at Port Klang was unhealthy while other parts of the country recorded readings between good and moderate.
API readings of zero to 50 indicate good air quality, 51 to 100 moderate, 101 to 200 unhealthy, 201 to 300 very unhealthy, 301 to 500 hazardous, and above 500 dangerous.
Meteorological Services Department principal assistant director Mohd Helmi Abdullah said the haze was caused by the Sumatra fires and the peat fire in Cyberpark, Sepang.
"The haze can be reduced only when the local fires are totally extinguished," he said.
"As the (monsoon) winds are blowing towards us, the smoke from Sumatra will continue to be blown our way."
Yesterday, 94 hotspots were detected in Riau, Sumatra, 58 in Borneo and nine in Selangor and Pahang.