[5] Hay then trained as a cardiologist in Britain, where he was influenced by the work of epidemiologist Sir Richard Doll linking smoking to adverse health outcomes.
[7] In Christchurch in 1958, Hay married Jocelyn Valerie Bell, whom he had met while they were both studying medicine at Otago.
[4] He was a long-time campaigner against smoking and the tobacco industry in his role as inaugural medical director of the New Zealand Heart Foundation (1977–92).
[3][6] In 1999, Hay retired from the New Zealand Heart Foundation after 30 years, having served as its president from 1996 to 1999.
[4][8] Hay held a range of positions on professional bodies, including vice president of the Royal Australasian College of Physicians from 1988 to 1992, president of the Canterbury branch of the British Medical Association in 1972, and New Zealand chair of the Cardiac Society of Australia and New Zealand from 1977 to 1981.