Ferns are useful houseplants for cleaning the air, particularly when it comes to formaldehyde. Formaldehyde is a problem in some newer homes. The emerging science of phytoremediation uses plants to filter toxins from air and soil. Studies revealed that ferns are a natural for absorbing formaldehyde from the air inside your home. As they transpire, the formaldehyde is taken into the plant with carbon dioxide, then only oxygen is released to filter air naturally. So if you haven’t repotted that old fern, or if you’re on a budget, buy lots of smaller ones and repot to make the air inside cleaner than ever.