The coroner who originally investigated Ella’s death ruled she had died of acute respiratory failure, but made no mention of pollution. Ms. Adoo-Kissi-Debrah did not know then what diesel fumes can do to young lungs. It was more than a year after Ella’s death that she first learned dirty air is a known asthma trigger. “It was like putting a picture together” as it finally began to make sense, she told me.

Air pollution has never appeared on a British death certificate, said Ms. Adoo-Kissi-Debrah’s lawyer, Jocelyn Cockburn. If a new coroner amends Ella’s to note its role, he or she could also demand that the government take action to prevent future deaths. And the moral and political repercussions could be even wider.

This grieving mother’s fight holds a power far greater than its potential to clarify the cause of one family’s tragedy. It’s bigger than just London and Britain, too. In demanding that dirty air be written into the official record as having contributed to her loss, Ms. Adoo-Kissi-Debrah wants to force us all to recognize a danger that is all around us, but which we have long chosen to ignore.

This danger is truly global, and it is a consequence of our decisions to remain dependent on dirty, deadly fossil fuels and our failure to force polluters — like Volkswagen and the other auto manufacturers whose brazen shattering of pollution limits has left so many Europeans breathing toxic fumes — to follow the rules.

It is not just that air pollution itself can be invisible. Its links to all manner of health woes — heart attacks and strokes, premature birth and dementia, among many others — while very real, are hard to make out. That is why getting it on a legal document as a contributing factor in the death of one child matters so much. The message would be unmistakable: This is not an abstraction.