The major harmful gas emitted by automobile vehicles is?

Answer 1

There isn't just one harmful gas that's emitted from an exhaust pipe -

Hydro carbons (HC - are unburnt fuel particles that attack the atmosphere)

Carbon Dioxide (Co)

Carbon Monoxide (Co2)

Oxides of Nitrogen (Nox)

There is also - But not so harmful -

Nitrogen (N2) and Oxygen (O2)

Bearing in mind that the normal Atmosphere 'Air' is made from -

78% Nitrogen, 21% Oxygen 0.94% Argon and 0.06% of other 'trace' gasses like Carbon Dioxide and Sulphur Dioxide (SO2) etc