It is known that exposure to carbon monoxide is harmful to animals because

1.  it reduces CO2 transport

2.  it reduces O2 transport

3.  it increases CO2 transport

4. it increases O2 transport   
        

(b) Haemoglobin consist of a protein globin and pigment here. The four portion of iron in name combine with molecule of oxygen. it is an easy reversible reaction to form oxyhaemoglobin. 
                                         Hb +  O     -     HbO2
where as the complex formed by the reaction of carbon monoxide and haemoglobin is incredible strong
                                      Hb   +  CO   -  HbCO
                             (haemoglobin)         (carboxy haemoglobin)
As a result of this strong between the haemoglobin and carbon monoxide the haemoglobin looses its affinity to oxygen thus may lead to choking or even death.