Delhi air quality is predicted to deteriorate to the "moderate" category by October 5 (File)

Air quality in the national capital slipped into "moderate" category on Thursday as wind speed was weak and directions became variable.

"Marginal deterioration of air quality to the middle of the moderate category is predicted for Friday. A change in predominant wind direction to the northwesterly, slight increase in wind speed in the influence of western disturbances is predicted by early October 5," according to the Centre-run System of Air Quality and Weather Forecasting And Research (SAFAR).

"Air quality is predicted to slightly improve but in the same moderate category and likely to stay in the same category for the next two days," it added.

The Central Pollution Control Board (CPCB) and SAFAR on Thursday recorded the city's air quality index in the "moderate" category -- at 187 and 105 respectively. On Wednesday, the city's air quality index was in the "satisfactory" category -- at 90 and 71 respectively.

An AQI between 0 and 50 is considered ''good'', 51 and 100 ''satisfactory'', 101 and 200 ''moderate'', 201 and 300 ''poor'', 301 and 400 ''very poor'', and 401 and 500 ''severe''.

Delhi's air quality is predicted to deteriorate to the "moderate" category by October 5 as a result of change in wind direction from easterly to north and northwesterly, SAFAR said, adding satellite data has indicated sporadic biomass burning signatures in north India over the last week but no increasing trend in fires counts has been noticed.