On the morning of the first school day this year, my son's form teacher surprised all parents of the class with her WeChat post of a sleek, new air purifier in the back of the classroom.

Thank-you messages, tearful faces, thumbs-up, flowers and other happy icons soon flooded the mobile forum. One mother said she was leaping with joy, and another claimed it was the best gift she had received for the Chinese New Year.

Then a father asked the teacher to turn the air to full blast, bringing to our attention the fact that it was a compact, family-use filter, which many smoggy days ago we had offered to donate by "putting together one day's or two days' lunch money", as one parent suggested.