On July 7, I travelled to school on a route I had never taken before, and have never taken since. I’m a design and technology teacher at a school in Westminster, and I was travelling from west London. But that day my usual Tube line was suspended, so I went to King’s Cross instead. I was about to get on a train, but I let it go and instead left the platform to check it was going in the right direction. Then I came back and got into a carriage with a bomber.

One minute you are sitting on the train, reading your paper, and the next minute a bomb goes off and all hell breaks loose. Everything gets blown out of your hands. You’re blown into the air, and then a pressure wave pushes you back into the chair. All the shrapnel from the bomb was flying everywhere. The doors and windows were blown out. The roof was ripped off the train, and two massive craters were blown in the floor.

It was dark, and I couldn’t breathe properly because of the smoke and chemicals from the bomb. When the fireball burst in my face, a thin layer of skin burnt off my eyes and my tear ducts dried up.

I got up, but I couldn’t walk - I started to fall over stuff, and couldn’t see what it was. Because I couldn’t see, I fell through a hole in the floor of the carriage. I was dangling there, about to hit a live rail. I knew what would happen if 450 volts went through my body: I would bounce up and down between the rail and the underside of the carriage. I didn’t really fancy that. But I was weak, and no one knew I was there. Fortunately, someone pulled me out.

I was covered in broken glass; one passenger took a lump of metal out of my head. But I knew that farther down the carriage, where the crater from the bomb was, people would be worse off.

The day before, I’d had a discussion with my Year 10 form about social responsibility. I had to practise what I had been teaching: I had a duty to go and help my fellow human beings.

I was crawling around on the floor, working out whether people were alive or not by feeling the pulse on their necks. It was dream-like. I remember thinking: “They haven’t cleaned this Tube very well.” I would put my hand on a seat and it would be covered in slime. But I couldn’t see what it was, so I carried on. I managed to save one guy. In the back of his leg, where his tendon was, he’d had a hole cauterised in the exact shape of a nail. He had lost 16 units of blood and I was trying to keep him awake. What do you say to somebody you have never met before?

It sounds crass, but I asked what he was doing at the weekend. He was playing Brutus in Julius Caesar, and I kept saying, “So, you’re playing Caesar?” to get a reaction from him. I didn’t leave the carriage until the first-aid person got a drip into him. At that point, I’d had enough, so I staggered out.

There was pandemonium outside: police, helicopters, cordons going up. I was covered in blood and soot and other stuff, but I just walked to school. There was a news black-out, so they weren’t sure what had happened. And I was just babbling.

I was a classic case of post-traumatic stress disorder. I was treated by an expert at a national trauma clinic. To recover, I had to go through everything all over again. It was ridiculously painful, but it enabled me to leave the traumatic situations behind.

Someone told me that the odds of my being in that carriage were maybe 65 million to one. How do you make sense of that? You don’t: you just have to accept that that is what happened. I made a decision that cost me dearly.

John McDonald was talking to Adi Bloom. He will be appearing before the inquest into the July 7 bombings on Tuesday. If you have an experience to share, email features@tes.co.uk.