To living organisms, the most dangerous debris from tephra is probably because it spreads so widely

The answer is volcanic ash.

Tephra is fragmented material ejected by an erupting volcano into the air. Volcano ash is a small fragment of tephra, and since it is light it can spread so widely. It is harmful to ecosystems. It may contain some gasses, such as carbon dioxide or fluorine, which are toxic to organisms, and thereby affect on crop failure, organism death, some deformity, and illness.

Because it's sugar that us the answer