Follow the instructions in your router's Users Guide to create a Guest wireless network (Section 4.7 of the user's guide.) This makes your router advertise a second wireless access point. Pick a different name for it, such as dharm0us_guest. Uncheck the box marked "Allow Guests to Access My Local Network". Pick a password for the guest network that is different from your home network's password. This essentially creates a DMZ, which is a network isolated from the rest of your home's computers and devices.
Then, reconfigure your Air Purifier to use the newly created Guest network, dharm0us_guest, with the new password. Now if your Air Purifier is hacked, it won't expose your internal computers to any threats.
Note that if you do this, you won't be able to "locally" control the air purifier with your smartphone app. Your only access will be via remotely, via their service interface. This may or may not be an issue for you, it's simply a consequence of isolating their device to reduce the risks to your network.
While you're at it, you might also want to follow the user's guide in section 4.9.4 to disable UPnP in your router. This will prevent these kinds of devices inside your network from opening security holes in your router without your knowledge.