Homeland Security Secretary Michael Chertoff stated the following towards illegal immigrants from Mexico: "Illegal immigrants really degrade the environment. I've seen pictures of human waste, garbage, discarded bottles and other human artifacts in pristine areas,...And believe me, that is the worst thing you can do to the environment.'' 

He made the statement before the AP defending the construction of the 670-mile long border fence along the US-Mexico border.

His comments basically states that Mexicans are dirty, care less about the environment, and cause damage to our natural resources.  And when Mexicans cross the border, they bring their qualities into our "pristine" nation. 

Although Mexicans do rely on dirty fuel to power their nation and do not have the environmental protective policies similar to the US, the illegal immigrants that do come to the US are hired to do the menial labor of keeping our nation "prestine."

http://www.miamiherald.com/418/story/262655.html