Sen. Mark Warner (D-VA), vice chairman of the Senate Intelligence Committee, speaks at the Council on Foreign Relations on June 17, 2019 in Washington, DC.

Days after Google promised to remove ads for medical face masks, promotions are still appearing next to online content related to the coronavirus, much to the ire of U.S. lawmakers.

Sens. Mark Warner (D-VA) and Richard Blumenthal (D-CT) sent a letter to Federal Trade Commission Chairman Joseph J. Simons on Tuesday, criticizing Google for continuing to serve ads next to stories about COVID-19 on various sites. Federal health officials have repeatedly told the public to refrain from purchasing masks so they can be reserved for people who need them, like health-care workers responding to the crisis.

"Google has made repeated representations to consumers that its policies prohibit ads for products such as protective masks," the letter said. "Yet the company appears not to be taking even rudimentary steps to enforce that policy."

The senators said that their staffers were able to find a number of ads, served by Google, for masks on news sites alongside stories about the coronavirus. On Wednesday, CNBC found additional mask promotions from other online ad companies, including Teads, RTB House, Criteo, Outbrain and Facebook Audience Network, appearing in or near stories pertaining to the coronavirus.

Google told CNBC last week that it was temporarily banning medical face mask ads, and said it would take a few days to remove them. Several days earlier, Facebook said it was temporarily banning ads and commerce listings selling medical face masks.

A Google spokesperson said in a statement that it has blocked "millions of ads that attempted to capitalize on coronavirus" and "implemented a temporary ban on all medical face mask ads."

An Outbrain representative said the company recently "strengthened its global advertiser guidelines to address ads promoting or implying the benefits of facemasks as a prevention or cure for the coronavirus." She said that Outbrain's guidelines "do not allow for medical mask ads of any kind on our network" and that "any advertiser that attempts to bypass our guidelines we will actively address and remove."

The other companies didn't immediately responded to requests for comment.

In addition to display ads, CNBC found examples of sponsored shopping listings for products that claim to prevent coronavirus, which Google has said are not allowed.

The rapid spread of COVID-19 is creating a major challenge for online ad companies that have been heavily criticized in recent years for allowing the spread of misinformation, largely related to political issues and elections.

YouTube said on Monday that, in removing videos that violate company policies, it will be relying more on technology to help with work normally done by human reviewers. That could lead to an increase in the amount of content taken down. YouTube also said it will use more automation for content moderation.

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