Having said that, I wouldn't necessarily trust anything coming out of the UN. They're not exactly known for objectivity as an organization.
You must realize that is very difficult to design reasonable scientific studies on the effects of second hand smoke. How can anyone quantify the amount of second hand smoke that he or she has been exposed to over a lifetime? Since any harmful effects on non-smokers would be intuitively correlated with the quantity of exposure, just about any of these studies would have a built-in flaw impossible to overcome.