Because/As the pullution is increasingly serious,we should take action to deal with it.
或者：The pollution is increasingly serious.,so we should take action to deal with it.
这两个句子之间只能是因果关系.
常用的其他关系;
条件关系可以用 if ,unless ,as long as .on condition that 等,
时间用:when ,while ,as ,as soon as ,before ,after ,the moment ,immediately ,since 等
让步关系用：though ,although ,even if ,no matter + 疑问词 等