Donald Trump and former Breitbart News executive chair Steve Bannon with a park ranger in Pennsylvania. AP Photo/ Evan Vucci Several ad tech vendors have told Business Insider they have received a flurry of requests from advertising agencies and advertiser clients to blacklist the right-wing news website Breitbart over the past month.

Breitbart, like many other sites on the web, generates much of its revenue by programmatic advertising. Ad tech companies provide automated systems to help advertisers target their desired audiences across multiple sites on the web, rather than needing to deal with each individual publisher directly.

Programmatic advertising is usually both cost-effective and time-effective, but it does come with the added risk that brands' ads can appear next to questionable content they wouldn't want to be associated with. To prevent this, most ad tech vendors allow advertisers to set up "blacklists" to stop their ads appearing on certain websites.

Some advertisers have taken issue with their ads appearing on Breitbart owing to the controversial and provocative nature of its content. The site has been accused of publishing content that is racist and sexist — a charge Breitbart denies — having previously run headlines including “Hoist It High and Proud: The Confederate Flag Proclaims a Glorious Heritage” and “Birth Control Makes Women Unattractive and Crazy."

One ad tech employee said Breitbart had become "the single most-requested specific non-porn blacklist site" they had seen in their career

Mani Gandham, CEO of martech firm Instinctive, said his company has received emails from three separate advertising agencies over the past week asking to blacklist Breitbart — and also other websites associated with reports around fake news — from all of their clients' ad campaigns.

For the most part, it seems these requests have been triggered by recent news reports of advertisers boycotting Breitbart. Kellogg's, Warby Parker, Allstate, Nest, and SoFi — plus ad tech firm AppNexus — are among the companies that have publicly stated they have pulled their ads from the website.

Breitbart, which claims to have 45 million "highly engaged, highly perceptive, highly loyal readers," (data from website analysis service Compete.com suggests the site has around 13 million unique monthly visitors) has responded by launching an online #DumpKelloggs campaign calling on its audience to stop buying Kellogg's products. The #DumpKelloggs petition states that "boycotting mainstream American ideas is an act of discrimination and intense prejudice."

Instinctive's Gandham said: "It seems that agencies only took action after the Kellogg’s fiasco and there were real consumer repercussions. It’s not that being reactionary itself is a bad thing, but there are also plenty of issues that should be preemptively handled and this is one of them. Now we’re seeing the fallout and companies are being hurt for both having ads run on these sites as well as pulling them off, putting them in a lose-lose situation."

Eric Franchi, cofounder of ad tech company Undertone, said his company also began receiving a steady rise in requests from clients to blacklist Breitbart following the news reports about Kellogg's.

Franchi said: "I can empathize with those at the senior levels of agencies and advertisers wanting to make certain there is no PR storm coming. We’ve seen this scenario happen time and time again over the years."

One ad tech vendor employee, who asked not to be named, said requests related to Breitbart had shot up in the past few weeks. They added this was "the single most requested specific non-porn 'blacklist' site" they had ever seen in their career.

Bill Lonergan, CEO of RadiumOne, confirmed his company had been approached by "some" advertisers to not include Breitbart as a source of impressions, but that he wouldn't describe it as a rush.

Brett Wilson, CEO of TubeMogul, told Business Insider: "Several of our brand clients have proactively blacklisted Breitbart. Since many of our clients use TubeMogul's software self-serve and have always had total control over the exact sites where ads run, they are frequently doing it in the software themselves rather than asking us."

A spokesperson for Rubicon Project also said the company offers its clients tools to control where their ads appear — and while it has clear guidelines for what is allowed within its marketplace (to exclude things like pornography and illegal content) it does not take an editorial censorship position.

The spokesperson said: "We believe that a commitment to a free and open internet is critical for the future of not only our industry but is also necessary to maintaining an open dialogue on the important issues facing the world at large. More broadly we also believe our industry has a responsibility to develop the highest quality advertising marketplace possible, focusing on clear vetting practices, standards and security measures from the very beginning of the process."

Will blacklisting hit Breitbart's revenues? And is it effective for advertisers?

It's unclear how much revenue Breitbart is potentially losing as it continues to be placed on advertiser blacklists. The company is private and does not disclose its financial numbers. Breitbart said last month Kellogg's decision "will make virtually no revenue impact," but now agencies are requesting the site is blacklisted across all of their clients, this could soon change.

A Breitbart spokesperson did not immediately respond to a request for comment.

That said, the nature of programmatic advertising means that if one buyer leaves the market, another soon fills its place in the auction that takes place for ad space every time a page loads. And Breitbart uses ad tech's largest player Google — through its ad exchange AdX and AdSense — to serve ads on its site, which millions of advertisers use every day.

Clicking on the "Ad Choices" logo on the right-hand corner of these two ads shows they were both bought using Google's ad tech. Breitbart.com/Business Insider

While blacklisting is one way to prevent a PR embarrassment, buying ads programmatically on the open web is inevitably a game of whack-a-mole. You can block one site, but it's only a matter of time before another site that doesn't meet a brand's values appears in the pool of ad inventory offered up by ad exchanges.

Gandham thinks blacklisting doesn't work and that even top advertisers with more than 12,000 web domains in their blacklists still can find themselves appearing on unsavory sites.

He added: "Having a trusted whitelist is the only sustainable method to get a baseline quality of inventory, with manually verified sites and continuous periodic review. It’s actually not that much work in the long run and can be shared amongst many clients and it isn’t a neverending chase like a blacklist. This still doesn’t account for all the various technical issues of programmatic that can render lists useless because of untrustworthy sellers and missing data but it’s a proper step forward."

Undertone's Franchi suggests a five-point approach for advertisers to ensure their brand safety: