as we all know ,smoking is harmful to our health as is known

其实没有必要去深究,只要记住：
非限制性定语从句：as we all know,smoking is harmful to our health,which is known to all.(关键在于health和is之间的逗号,有逗号的话就叫做非限制性定语从句,而且连接词用which,who等,但绝不能用that)
限制性定语从句：as we all know,smoking is harmful to our health as is known to all.(这个health和is之间没有逗号,这样叫做限制性定语从句,连接词只能用as).