Water filters come in all shapes and sizes these days—think carafes, bottles, and jugs. But you can’t judge one based on how aesthetically pleasing it is, or how well it will match your new vintage rattan coffee table. The best water filter pitchers will be certified by NSF International or the Water Quality Association, which are independent, third-party organizations that test the efficacy of water filters, according to the Environmental Working Group (EWG).

The EWG recommends you start by looking at their Tap Water Database or your local water utility’s website to learn which particular contaminants are in your tap water before choosing a filter. There are many different water filtration options out there, but according to the EWG they all rely on on of the following methods: carbon or activated carbon, reverse osmosis, deionization, ion exchange, mechanical filters, ozone, ultraviolet and water softeners.

The site adds that carbon filters tend to be the most popular because they are affordable and effective at reducing levels of common water contaminants like lead. These systems pull out the toxins in the water, which is then followed by another stage so that you don’t consume the charcoal, naturopathic doctor Gabrielle Francis, DC, previously told Well+Good. “Many use carbon blocks, which don’t take out the good minerals in the water—only the bad,” she says. Another popular option is reverse osmosis filters, which remove things carbon filters aren’t able to (like arsenic), but these filters tend to cost more.

No matter what type of water purifying system you choose, it’s important to replace your filters frequently. “The whole idea of a water filter is to remove chemical contaminants—they’re not all that great at removing microbial contaminants,” Jason Tetro, a microbiologist and author of The Germ Files, previously told Well+Good. He suggests using your sense of smell and taste to determine when you need to replace your filter—if you pour yourself a glass of water and it smells weird and/or tastes yeasty, musty, or fishy, it’s time to swap out your filters.

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