Just the 80 dollar temtop that sells on Amazon. Easy enough to check against local purple air sensors for the particulate readings. I did that, and they were spot on for my neighborhood. Don’t know if I trust the formaldahyde reading but the tvoc corresponds to my physical experience very well, and it drops to near 0 tvoc when taken outside. If there’s a catch, it’s that you don’t know what voc you’re dealing with. Could be something lethal like radon or something fairly innocuous at the levels I’m seeing. Whatever it is in my case, it causes very predictable migraine symptoms which correspond to the voc level almost perfectly.