It really is difficult to say.
Mind you, asthma, eczema, hayfever, allergies have very very much a north-south connection, a weather thing as well.
While in NZ, Scotland and Kanada 37% of the children suffer from asthma, in Germany it's 10%. In Bulgaria, Israel or Turkey it's below 5%. The climate has a huge effect, so only really countries with the same climate can be compared in methods of allergy prevention.