Brian Whatcott wrote:
Nitrogen displaces oxygen, so you pass out without warning. It is not
otherwise toxic. So figure being underwater for a few seconds. That's
as much time as you have with N2. With CO2 you pant.
Both gases diffuse of course.

"Carbon dioxide is a colourless gas which, when inhaled at high
concentrations produces a sour taste in the mouth and a stinging
sensation in the nose and throat. These effects result from the gas
dissolving in the mucous membranes and saliva, forming a weak solution
of carbonic acid."

Material Safety Data Sheet
Gaseous Nitrogen:

SIMPLE ASPHYXIANT
Section 3 : HAZARD IDENTIFICATION
Emergency Overview:
Nitrogen gas is colorless, odorless and non-flammable.

It is non-toxic. The primary health hazard is asphyxiation by
displacement of oxygen.
Route of entry:
Inhalation, skin and eye contact.
Effects of acute exposure
Eye contact:
Vapor may cause a stinging sensation.
Skin contact:
No adverse effects from gas.
Inhalation:
May cause dizziness.
Asphyxiant.
Can cause vomiting.
May result in unconsciousness.
May cause excitation, excess salivation, rapid breathing.
May cause headaches and drowsiness.
May cause stinging of the nose and throat.
Ingestion:
Not a likely route of exposure.
Effects of chronic exposu
None known. Chronic exposure to abnormal concentrations unlikely
Reproductive effects:
Oxygen deficiency during pregnancy has produced developmental
abnormalities in humans and experimental animals.