막연히 중국산 건식벽체에 뿌려진 훈증약이나 원료로 사용된 '플라이 애시'(flyash)라 불리는 석탄재가 원인이 아닐까 추정할 뿐이다.
The reason is suspected to be coal ash, sometimes known as "fly ash," which was utilized as a fumigation or raw material sprayed on Chinese dry walls.