Question

what is permissible levels of emission of various gases

Answer 1

The permissible levels of emission of various gases refers to the standards and regulations set for the amount of pollutant gases allowed out of a given engine or automobile in a given period of time. These standards vary from country to country and even in the same country, there might exist different standards for various regions. The standards also vary in terms of the type of engine and fuel used. (these regulations apply for both vehicles and industries)
Here is an example using the Bharat stage emission standard with references made  to the Euro 6 standards:
For trucks and buses: CO= 4.0,HC=0.55, NOx = 3.5, PM= 0.03
Light duty petrol vehicles ( 4 wheels): CO=2.27, HC=0.16, NOx= 0.11
3-2 wheel vehicles: CO= 1-1.25,HC+NOx= 1-1.25
Note: These standards vary from region to region and also in regards to the type of engine used.

Answer 2

All motor vehicles release smoke which has the potential to pollute the environment, if left unchecked. As such, there have be some norms to check the vehicular emission levels to determine how safe they are and whether they add to the city’s air pollution.

Permissible levels of emission of various gases.

TYPE OF VEHICLE % OF CARBONMONOXIDE HYDROCARBON MEASURED IN PPM

2 and 3 wheelers (2 - stroke) manufactured after 31st March 2000 3.5 6,000

2 and 3 wheelers (2/4 - stroke) manufactured on or before 31st March 2000 4.5 9,000

2 and 3 wheelers (4 - stroke) manufactured after 31st March 2000 3.5 4,500

4 wheelers manufactured according to Pre Bharat Stage II Norms 3 1,500

4 wheelers manufactured according to Pre Bharat Stage II, Stage III or subsequent norms 0.5 750

Explanation: