Efficacy of indoor air purification in treating Artemisia (mugwort) pollen allergic rhinitis: study protocol for a randomised controlled trial oleh Qiao-yan Chen, Li Li, Li Zhang, Jin-han Mo, Zi-feng Yang, Xiao-lin Wei, Yun-ying Li, Ji-yan Xia, Xiao-bing Bai, Pei-fang Xie