Humans have been filtering water for millennia. In fact, it was the Greek physician, Hippocrates, who developed one of the earliest filters: a fabric bag that was designed to remove the sediments that made water smell or taste bad. Fortunately, things have become a little easier since then with the introduction of faucet filters. Here are three things to check out when buying.

Before buying anything, check the faucet that you want to use the filter with. You’ll find that most filters are designed to fit standard faucets only, which means if you have a pull-out, hand-held, or spray-style faucet, you won’t be able to attach the filter properly. And it’s important that these types of filters screw on securely and that there’s a decent seal between the faucet and the filter—otherwise, you’ll get leaks spraying all over the place.

Remember to factor in the cost of cartridges when buying a faucet filter. The devices themselves may be relatively inexpensive, but to continue getting the benefit from them, you’ll need to replace the cartridge on a regular basis. Most cartridges need to be replaced every 100 gallons, roughly every three to four months—many also come with an indicator to let you know when it’s time to change.

Most faucet filters have a switch that lets you divert water from the faucet via the filter. This is especially important if you have a mixer faucet, as hot water shouldn’t go through the filter. It’s also worth bearing in mind that filtration systems like this often can’t take the volume of water that your regular faucet can produce, so you’ll have to be careful not to open it on full when using the filter. Otherwise, you can end up with leaks or put unnecessary pressure on the device that, over time, can cause it to crack.