Upon contact, white phosphorus can burn people, thermally and chemically, down to the bone as it is highly soluble in fat and therefore in human flesh. White phosphorus fragments can exacerbate wounds even after treatment and can enter the bloodstream and cause multiple organ failure. Already dressed wounds can reignite when dressings are removed and the wounds are re-exposed to oxygen. Even relatively minor burns are often fatal. For survivors, extensive scarring tightens muscle tissue and creates physical disabilities. The trauma of the attack, the painful treatment that follows, and appearance-changing scars lead to psychological harm and social exclusion.

Just to reiterate, cuz it really sounds like some sci-fi alien shit:

Already dressed wounds can reignite when dressings are removed and the wounds are re-exposed to oxygen.