12 min read
TL;DR
- Blocklists fail on context, not on judgement. A film shoot story gets blocked for the word shot. Coverage of the band Arcade Fire gets blocked for the word fire.
- Lists also decay. Brands that added Paris after the 2019 Notre Dame fire were still blocking Paris during the 2024 Olympics, because nobody went back and removed it.
- Safety and suitability are different standards. The industry framework separates a floor of universally harmful content from graded risk levels that are a matter of brand preference. Most blocklists collapse the two.
- The audit is four steps and starts with exporting your own exclusion list, which most buyers have never actually read.
Table of Contents
Almost every programmatic campaign runs against a brand safety blocklist, sometimes called a keyword blocklist or ad exclusion list. Almost nobody has read theirs.
They tend to arrive as a vendor default or get inherited from whoever set the account up, then quietly grow as terms get added after an incident and never removed. Nobody is accountable for the list itself, which means nobody audits it, which means nobody knows what it is currently costing.
There is now a public number for what that costs, and it is larger than most buyers would guess.
What a Keyword Blocklist Actually Does
A keyword blocklist is a list of terms. If a page or URL contains one, your ad does not serve there. That is the entire mechanism.
It has no model of meaning. It does not know whether the word appears in a report on a shooting or a report on a film shoot. Industry lists typically run to thousands of terms and include words like kill, death, shoot and fire, as the Reynolds Journalism Institute has documented.
The appeal is obvious. It is cheap, it is instant, and it produces a defensible answer if anyone asks whether you took precautions. The cost is invisible, because a blocked impression does not show up anywhere as a loss. You simply never see the inventory you did not buy.
What the Reuters Test Found
In 2026 Integral Ad Science and Reuters ran a test together and published the results, which is unusual. Verification vendors and publishers do not normally co-author a critique of a tool the vendor sells.
The method matters, so here it is. The study looked only at Reuters content that IAS had already classified as low or medium risk under its own brand suitability measurement, meaning inventory its contextual targeting product would have cleared and that would have qualified for brand suitable private marketplace deals. Then it asked a simple question: how much of that already cleared inventory would a typical keyword blocklist have blocked anyway?
As reported by AdExchanger, the answers were:
- News section, tested in June: 54 percent of URLs cleared by contextual targeting would have triggered a typical keyword blocklist.
- Lifestyle section, tested in September: 4.27 percent of cleared URLs would have triggered blocking.
- On that lifestyle section, those pages carried 13.5 percent of the section’s ad impressions during the test period.
That third number is the one to sit with. A small share of URLs can carry a large share of impressions, because traffic is never evenly distributed. Blocking four percent of pages does not cost you four percent of reach if the blocked pages happen to be the popular ones.
Two caveats stated by the researchers themselves, which are worth repeating rather than burying. The study did not run test campaigns, so it measured what would have been blocked rather than observing live ad serving. And on the news side, the 54 percent figure describes URLs rather than impressions, so it is not a claim that half of Reuters news impressions would have vanished.
The two ways lists fail
The examples from the study are worth knowing because they show the failure is structural rather than occasional.
Failure one is context. A Reuters lifestyle article about a film shoot for The Devil Wears Prada 2 would have been blocked because the URL contained the word shot. Coverage of the band Arcade Fire gets caught by the word fire. Neither page is unsafe by any definition anyone would defend out loud.
Failure two is decay. This one is more expensive and gets discussed less. Reuters Digital general manager Phil Andraos described how blocking a term tends to pull associated terms in with it. After the 2019 Notre Dame fire, brands blocking fire also added Paris. Blocklists are not reviewed on any schedule, so those same brands were still blocking Paris five years later, during the 2024 Olympics, which is the single largest advertising event in the calendar.
Nobody decided to block the Olympics. The decision was made in 2019 about a cathedral, and then nobody revisited it.
The 2019 Number, and the Argument About How Common This Still Is
The Reuters test is recent. The scale estimate is older and comes from a 2019 study by the cybersecurity firm CHEQ, conducted with University of Baltimore economist Roberto Cavazos, titled The Economic Cost of Keyword Blacklists for Online Publishers.
Its findings, as reported by MediaPost at the time:
- US publishers lost an estimated 2.8 billion dollars in 2019 to incorrect blocking, roughly one in every four dollars of programmatic news ad revenue, against an estimated 12 billion dollars spent annually on US online news advertising.
- Only 9 of the 100 most read New York Times articles that year were considered brand safe by keyword blocklists.
- 80 percent of ads served to premium publishers were subject to keyword blocklists.
- The distribution was uneven. Digiday reported that on the LGBTQ publisher PinkNews, 73 percent of safe articles were blocked, driven by terms like lesbian and same sex sitting on standard industry lists.
Here is where an honest article has to slow down, because the industry does not agree on how much of this is still happening.
IAS chief product officer Srishti Gupta told AdExchanger that the share of ad impressions running through the IAS platform blocked as a result of the exclusive use of keyword blocking is now just 0.25 percent, and that education has made the practice less popular over time. Andraos, on the publisher side, said advertisers appear to use extensive keyword blocklists in the majority of cases, and that the impact on revenue is significant for news and non news publishers alike.
Both statements can be true, and the gap between them is instructive. The IAS figure measures campaigns relying on keyword blocking alone. Gupta noted separately that many advertisers still use some form of keyword blocking even when they have also adopted more sophisticated tools. So a campaign running semantic contextual targeting plus a legacy keyword list would not appear in the 0.25 percent, while the legacy list keeps blocking things underneath.
Which is precisely why this is worth checking on your own account rather than assuming the industry number covers you.
Safety Versus Suitability, and Why the Distinction Matters
The industry solved this conceptually years ago. Most buying practice has not caught up.
In 2019 the Global Alliance for Responsible Media published its Brand Safety Floor and Suitability Framework. As IAB Tech Lab describes it, the framework maps eleven content categories against four risk levels, low, medium, high and floor, and is activated through the Tech Lab’s content taxonomy.
The distinction it draws is the useful part:
- The floor is content no advertiser should appear beside. That is a safety question and it is not a matter of taste.
- Everything above the floor is graded risk, and where a given brand draws its line is a preference. A funeral insurer and a theme park will reasonably answer differently about the same page.
A keyword blocklist cannot make that distinction, because it operates on strings rather than categories. Every term on it is treated as floor level regardless of what the brand actually thinks.
One piece of context worth knowing if you are looking this up. GARM was discontinued on 8 August 2024, following a US House Judiciary Committee investigation and antitrust allegations raised by X. The organisation is gone. The framework it produced remains in use, embedded in verification platforms and in the IAB Tech Lab taxonomy, and the Brand Safety Floor is incorporated by reference into the Media Rating Council’s supplemental guidance on content level context and brand safety verification. So the standard did not die with the body that wrote it.
The Framework: Auditing Your Own Exclusion List
Four steps. None of them require new budget, and the first one is the one most teams have never done.
1. Export the list and actually read it
Why it matters: you cannot evaluate a policy you have never seen. Lists inherited from a vendor default or a previous agency represent decisions nobody at your company made.
How it is done: ask your DSP or verification partner for the current active exclusion list as a file. Read it top to bottom. Expect it to be longer than you think and to contain terms nobody can explain.
2. Separate genuine risk terms from news and language terms
Why it matters: this is the safety versus suitability line applied to your own list. Some terms genuinely map to floor level content. Others are ordinary English that appears in sport, entertainment and current affairs.
How it is done: sort every term into keep, review or remove. Shoot, fire, shot, attack, death and dead almost always belong in review, because their presence in a URL says nothing about the content. Place names added after a specific event are the clearest removal candidates, per the Paris example.
3. Ask your verification partner what your blocking actually costs
Why it matters: the industry average tells you nothing about your account. What you want is your own blocked impression volume, and how much of it was blocked by keyword rules rather than by category classification.
How it is done: ask for a breakdown of blocked impressions by cause. Vendors have built tooling for exactly this, including products that give publishers visibility into how buyers are blocking them so overblocking can be diagnosed. If your partner cannot produce the split, that is itself a finding.
4. Give the list an owner and a review date
Why it matters: decay is the failure mode that costs the most, and it happens entirely through neglect. Terms get added under pressure during an incident. Nothing ever triggers their removal.
How it is done: name a person, set a quarterly review, and record the reason next to any term added reactively. A term added because of a specific event should carry the date of that event, so the next reviewer can see it is five years old.
None of this is an argument against brand safety. A floor exists for good reason, and no serious advertiser wants to fund content that sits below it.
The argument is narrower. A list that nobody has opened in two years, inherited from a vendor default and grown by reflex after incidents nobody remembers, is not a brand safety policy. It is a leftover, and it is quietly deciding where a meaningful share of your budget cannot go.
Export yours this week and read it. Most people find something in the first five minutes that nobody can justify.
FAQ
Brand safety concerns content no advertiser should appear beside, described in the industry framework as the floor. Brand suitability is graded above that floor and is a matter of individual brand preference. A keyword blocklist cannot tell the two apart, because it matches strings rather than assessing content.
In a 2026 test by Integral Ad Science and Reuters, 54 percent of Reuters news URLs already cleared as brand suitable would have triggered a typical keyword blocklist. On the lifestyle section the figure was 4.27 percent of URLs, but those pages carried 13.5 percent of the section’s ad impressions during the test period.
Because terms are added reactively during incidents and almost never removed. Reuters described brands that added the word Paris after the 2019 Notre Dame fire and were still blocking it during the 2024 Olympics, since the lists are not reviewed on any schedule.
The industry disagrees on this. IAS says only 0.25 percent of impressions on its platform are blocked through the exclusive use of keyword blocking, while also acknowledging that many advertisers still run some keyword blocking alongside more sophisticated tools. Reuters says advertisers appear to use extensive blocklists in the majority of cases. Checking your own account settles it.
The organisation was discontinued on 8 August 2024 following a US House Judiciary Committee investigation and antitrust allegations from X. The Brand Safety Floor and Suitability Framework it produced remains in use across verification platforms and the IAB Tech Lab taxonomy, and the Floor is incorporated by reference into Media Rating Council guidance.
