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In 70% of states across the country, women are charged a sales tax when they buy feminine hygiene products.

Every state has its own list of items deemed “necessary” that are tax exempt, ranging from lip balm and dandruff shampoo and condoms, to pumpkins specifically for eating in Pennsylvania and gun club memberships in Wisconsin.

But aside from a handful of states, tampons and sanitary napkins are not included in this list of so-called “necessary” items. That means that women end up spending an estimated $150 million a year on the sales tax for these items.

A number of countries around the world have already stopped taxing menstrual products because of the unfair financial burden it puts on women. Kenya led the way in 2004, followed by countries like Canada, Malaysia, India, and Australia. Absent from this list? The U.S.

And even among the 15 states that don’t currently tax feminine hygiene products, five simply don’t charge a sales tax at all and five eliminated the tax after advocacy against it. Just five states—Maryland, Massachusetts, Minnesota, New Jersey, and Pennsylvania—have these products marked as necessary, tax-exempt items. The cities of Chicago, Denver, and Washington, D.C., also don’t tax menstrual products.

Period Equity, a non-profit, and LOLA, a reproductive health brand, are partnering to change that. On Tuesday, they announced the launch of Tax Free. Period., a campaign to mobilize legal action to end the tampon tax in the 35 states in the U.S. that still charge it by Tax Day 2020.

“People get periods. Half of the population at some point in their lives, in fact,” the campaign explains. Noting that more than half of U.S. states don’t consider period products necessities, Tax Free. Period. argues that “as people with periods, we know tampons and pads are necessities. And we shouldn’t be taxed because of our biology. We’re calling it discriminatory.”

“Challenging the tampon tax in the U.S. is about much more than lifting a simple financial burden,” added Jennifer Weiss-Wolf, co-founder of Period Equity. “It is a way to call out laws that are archaic, unfair, and discriminatory. It helps us move toward a better model of economic parity and gender equity. And we believe it is a gateway for getting people to talk and think about the wider implications of menstruation—social, economic, and otherwise.”

Is your state taxing your period? News flash: there are 35 states that charge sales tax on our period products. We are calling that illegal, discriminatory, and unconstitutional. Join our legal fight at https://t.co/2rZHRTGfJL.

— LOLA (@mylolatweet) June 11, 2019

Their approach is threefold: first, the Tax Free. Period. website will serve as an educational platform, as well as an opportunity for individuals to take a pledge and get state-specific information and prompts to contact governors and legislators to pressure them to reverse the tax.

Period Equity and LOLA will also be amplifying the message on social media alongside thought leaders and celebrities and encouraging others to get involved and join the conversation.

Athlete and LOLA investor Serena Williams is on board, telling Fortune, “A tax on periods is wrong. Telling half of the population that their needs aren’t important is wrong.”

Finally, there will be a legal hackathon at Columbia Law School in the fall to “bring together top legal minds in constitutional, taxation, and sex discrimination law” to devise a legal strategy to get rid of the tampon tax in the 35 states that still have it.

Tax Free. Period.’s ambition may not seem to be a challenge on first glance. But 32 states have introduced bills to make menstrual products tax-exempt since 2016 and these efforts have failed—in many cases, more than once.