"Try to imagine it like this," he says, "the world's worst ever hangover. When they let me out of hospital, I slept for a few hours and then woke up with a raging headache. I couldn't focus on close objects, and my pupils were like pinpricks - my kids nicknamed me the Alien. Think of the worst hangover you've ever had, and then add a bit more. This went on for a fortnight. Not a pleasant experience."
A year ago this morning Mr Pearson, in his own words, "entered the history books for a few seconds" when he became the only British victim of the Tokyo subway attack. Just after 8am on 20 March 1995, he was commuting from his family's apartment in western Tokyo to the offices of Westpac Banking Corporation, the Australian company where he is chief manager. After one stop, people in his carriage started coughing. Mr Pearson noticed "a faint sweet, plastic kind of smell".
The train was crowded and at the next station he had to step off to make way for disembarking passengers. "It was then that I realised that something was wrong. They were carrying an old man off the carriage in front and laying him on the platform. He was lying there with his arms and legs convulsing, and there were people all around him sitting on the floor looking very unwell."
The old man died soon after, one of 12 victims of the most bizarre terrorist attacks in Japan's history. Half an hour before, 10 members of the Aum Shinri Kyo cult had boarded five separate trains, carrying plastic bags filled with the liquid form of sarin nerve gas. Travelling in pairs, they punctured the bags with sharpened umbrellas, before jumping off.
The cult's leader, Shoko Asahara, who finally goes on trial next month, had for years been predicting an apocalypse and acquiring chemical and biological weapon ingredients with the aim of hurrying the end along. Apart from the 12 dead, 5,500 people were poisoned by the sarin. Three remain unconscious, and will probably never wake up.
Mr Pearson was lucky. Somehow he struggled into a cab and made it to the office. His colleagues took him to hospital.
Within two weeks, he returned to work. But a month after the attack, he woke up in the night, trembling and hyperventilating. The panic attacks came every couple of weeks. According to Kanzo Nakano, a psychiatrist, 20 per cent of the sarin victims still suffer from headaches, fatigue, dizziness, nightmares, claustrophobia, flashbacks, depression and suicidal thoughts.
"Even last month, three new patients came forward for the first time," says Dr Nakano. "The long-term after-effects are immense, like a bottomless pit."