Donna Summer 9/11 Gave Me Cancer

Donna Summer was convinced -- inhaling toxic air after 9/11 gave her the lung cancer that eventually killed her ... TMZ has learned.

Sources close to the singer tell TMZ what we were hearing this morning -- that Donna was in New York City during 9/11, living at an apartment near Ground Zero.

Donna became almost paranoid about breathing the air, which was heavy with a rancid odor. 

In the months and years following 9/11, Donna's feelings intensified.  One source tells us when he was around Donna, she would constantly spray some sort of disinfectant in the air.  Deney Terrio, the host of "Dance Fever," tells us ... when he was around Donna post 9/11, she would hang silk sheets in her dressing room to prevent dust from coming in.

And one source says ... Donna, who was a fervent practicing Christian, believed 9/11 was an attack on Christianity and in some metaphysical way Christians like her were targets.  She somehow felt that her illness was a byproduct of the attack.

And, we're told, after Donna was diagnosed with lung cancer, several people told her that cigarette smoke may have been the culprit -- she was a smoker, and she also frequented clubs where people smoked.  But Donna simply didn't buy it.