American investigative journalist Seymour Hersh claimed yesterday there is no evidence that the Assad regime has been using chlorine as a chemical weapon.

Numerous chlorine attacks have been documented in Syria and according to OPCW investigators the chemical has been used "systematically and repeatedly".

Interviewed on the BBC's Daily Politics programme, however, Hersh made the astonishing assertion that "there's no such thing as a chlorine bomb". 

"The American intelligence community has been very clear that there's no evidence," he said.

At first it sounded as if Hersh was simply making a technical point – that chlorine weapons aren't "bombs" because they don't contain explosives – but as the interview continued it became clear that he was not. 

He went on to suggest the reported deaths and injuries were a result of airstrikes hitting stores of chlorine on the ground: 

"Chlorine is the basic way in Syria, the basic way once you are outside of the major city of Damascus where you purify water, and it's kept in canisters. They can easily be opened up when a bombing takes place. They're everywhere ... When chlorine's there in a small area, like in any place in a small area with no windows, you are going to die."

For general sanitation purposes chlorine is normally in liquid form, but evidence from reported chemical attacks points to the use of chlorine gas in pressurised cylinders which are dropped from the air and burst open on hitting the ground. While the Syrian government and its ally, Russia, have been keen to deny such attacks, neither of them have put forward Hersh's explanation that accidental spillages of liquid chlorine are to blame.

Hersh put forward a similar theory in connection with the sarin attack in Khan Sheikhoun last year, claiming the deaths occurred when Syrian forces using a conventional bomb accidentally hit a store of "fertilisers, disinfectants and other goods" causing "effects similar to those of sarin". Scientifically-speaking, this was nonsense and laboratory tests later confirmed the presence of sarin.

Hersh became famous for his exposé of the Mai Lai massacre in Vietnam back in 1969 and later exposed the horrors of Abu Ghraib prison in Iraq. More recently, though, he has written several error-strewn articles disputing the use of chemical weapons in Syria.

Below is a transcript of the relevant part of his BBC interview:

Presenter: You've questioned whether Bashar al-Assad was responsible for chemical weapons attacks in Syria last year. I know that you like to challenge orthodoxies and you suspect that governments might be covering up, but a UN report concluded it was confident Damascus had used the nerve agent sarin in the most recent attack. Who are you to disagree with those conclusions?

Hersh: Well, you're not quite quoting it right. There was a story in the New York Times that there was a section of the report that was deleted because of the lack of confidence. There was a story just the other day, again about the Assad regime dropping chemical bombs – chlorine bombs – and there's no such thing as a chlorine bomb. There's just so many factual things that people say that aren't accurate.

Chlorine is just a gas. It's not a chemical warfare weapon. You smell it, you run away. You can't drop a bomb on it because it would immediately burn up – it's a very reactive chemical. And by the way there's been studies showing ... I'm just talking facts.

Presenter: Well, you say they are facts but there is mounting evidence from neutral observers on this to onfirm that chlorine was used. The World Health Organisation said that local authorities in Douma where the attack is supposed to have taken place confirmed that on the day of that alleged bombing they treated 500 patients with the symptoms of exposure to toxic chemicals. Is that not strong enough evidence?

Hersh: Chlorine is the basic way in Syria, the basic way once you are outside of the major city of Damascus where you purify water, and it's kept in canisters. They can easily be opened up when a bombing takes place. They're everywhere.

All I am saying to you is that there were stories saying that the Syrians also used nerve gas in connection with chlorine gas – I wrote a book about this in 1968 – that the government, the American intelligence, the American Chemical Corps tested – chlorine's very reactive, it's a very reactive chemical ...

Presenter: Well, we saw those pictures and there's evidence of – are you saying that's fake?

Hersh:  No, I'm not saying they are fake. Chlorine – when chlorine's there in a small area, like in any place in a small area with no windows you are going to die.

Presenter: Are you saying it was used?

Hersh: They tested, in the Fifties, they tested chlorine with nerve agent to see how – whether the chlorine would supe it up. In fact what the chlorine did is it grabbed all the hydrogen molecules and diminished it. There's just no way you can use sarin and chlorine, as was written about all the time. I'm just talking about facts.

Presenter: But do you think chlorine was used in this instance?

Hersh: Chlorine exists all over the place ... there's intensive bombing going on by the Syrians, Russians are bombing. We are talking about an area that's been in rebel control for ever ...

Presenter: It's a weapon, to hurt and kill people?

Hersh:  Bombs frequently ...

Presenter: I'm trying to pin you down here, Seymour Hersh, on saying that you questioned whether Assad was responsible. Is there any credible counter-evidence?

Hersh: All I can tell you is that the American intelligence community report – I wish I could flash it here – but the American intelligence community has been very clear that there's no evidence that the Russians, that the Syrians, the regime used a chlorine weapon because there is no such thing. Chlorine exists. You bomb. Chlorine gets out there.

As you know, in East Ghouta, the opposition, they were in a dead-end game ... the people, ISIS, refused to let many of the civilians leave ... this is not a brief for Assad.

Presenter: Aren't you? You sound like an apologist for Assad.

Hersh: I just believe in facts. 

Presenter: But are they facts? It all seems to be very debatable. Are you making an apology for Assad and as an extension of that for Putin?

Hersh: The facts are that like a lot of leaders in the Middle East, in Saudi Arabia, in Bahrain, they can be very brutal ... he's in the middle of a war and it's very brutal – it's a horrible process. Ask the Americans how we do our wars. We do wars. Ask the Russians how they do the war in Chechnya?? It's always awful.

All I am saying to you – there's no credible American intelligence report that suggests that Bashar Assad dropped chlorine bombs. There's just no such thing. That's all I'm saying.