Smelling the sweet, pungent, fruity scent of isocyanates is an indication you're over exposed though you cannot rely on smell alone. The raw materials that make up all polyurethane products are designed to create synthetic rubbers, foams, and weather-proof coatings such as the paint on your car. Though the chemical compound of isocyanates play an important role in manufacturing, they are extremely hazardous when inhaled.
The major hazard with spraying polyurethane paints is breathing the mist or aerosol droplets and vapours of the paint spray and inhaling the isocyanate and other components into your lungs.
Sensitization may develop as a result of a large single overexposure, for example, from a spill or accident, or from repeated overexposure at lower levels. Once sensitized, these people, when later exposed to even very low concentrations of isocyanates even at levels below the exposure standard, can react by developing asthma-like symptoms, such as chest tightness, cough, wheezing and shortness of breath.
The issue with use of air purifying respirators is that they will reach a point at which the filter becomes saturated and will no longer capture the isocyanate or other solvents. When that filter breakthrough happens, an overexposure can occur, potentially causing an irreversible sensitization. Use of a supplied air respirator removes this filter change factor - it does not rely on the painter changing their gas/vapour cartridges and filters at appropriate intervals.