Sinha has been a passionate campaigner for justice for the victims of the Bhopal disaster since 1993, when he created the first advertisement for the Bhopal Medical Appeal (using the now-famous photograph by Raghu Rai of a dead child being buried) that raised money to build a clinic to provide free treatment for the survivors. He is an outspoken critic of Dow Chemical Company, the multinational owner of Union Carbide, whose neglected, dilapidated and undermanned chemical plant in the city of Bhopal leaked 400 tonnes of poisonous gas on the night of 3 December 1984, killing up to 8,000 people and injuring perhaps half a million. Since then, 20,000 have died as a result of injuries sustained on "that night", and more than 100,000 remain chronically ill; the abandoned, derelict factory continues to leach toxic chemicals into the groundwater, poisoning wells.

Apart from his novel, Animal's People (a fictionalised account of the aftermath of the disaster), Sinha has given many interviews and written several articles on the subject, including one in The Guardian marking the 25th anniversary of the disaster, in which he roundly condemns local politicians (particularly Babulal Gaur of the BJP) for their failure to comply with a 2004 Supreme Court ruling ordering them to supply clean water to the poison victims. Nor does he absolve the Indian Government of blame.

As a former advertising copywriter, Sinha is particularly scathing about Dow Chemical Company's attempts to improve its image through its sponsorship of events such as the Blue Planet Run and Live Earth Run For Water. He has called Dow's "Human Element" advertising campaign "glossy falsehoods", and quotes artist Paul Phare (who created a set of e-cards in response to Dow) “Telling a lie beautifully does not make it true.”