As a whole air purifiers are not harmful. The standard fan/HEPA filter/carbon filter air purifier is very safe. Those that generate ozone are not safe. Very few air purifiers emit ozone, none of these standard and/HEPA filter/carbon filter air purifiers do. It is always a small possibility with electronic air purifiers that are either poorly designed or malfunctioning. You could look for California Air Resources Board certification as if the device is certified it will not be emitting significant amounts of ozone. Dehumidifiers are not dangerous, the condensate collection tank needs emptying every so often and if it is not then bacteria and fungi could grow in the condensate.