Air purifiers and how to choose the right one..help plz?

Hey guys!

Im asking this question because my mom was told that she needed to buy an air purifier for my tittle brother to relieve his asthma symptoms. However, we have looked online and to be honest we’re not sure which one to get or what we need to look out for when comparing different brands. If someone could explain to me what is the difference between different air purifiers or even suggest a particular one we could buy, I would really appreciate it. Thanks