These substances have been proven to cause the following adverse effects:

• Benzene - a known carcinogen; causes cancer; long-term exposure to high-levels in the air can cause leukemia and cancer of the blood-forming organs; can damage the immune system.3
• 1,3-Butadiene - probable carcinogen (tumors found in animal studies due to exposure); can cause central nervous system damage; causes ear, nose and throat irritation.4

These are links to the health hazards of Olefins that can be found in refinery gas:

Answers.com - Benezene   

 U.S. EPA - Propylene oxide   

 Propane Gas Association of Canada - Toxic Substances   

 Berkeley University of California - The Carcinogenic Potency Project   

 U.S. National Library of Medicine - Toxicology Data Network   

 U.S. National Library of Medicine - Human Health Effects   

 U.S. National Library of Medicine - Haz-Map   

Tox Town - Enviromental Health Concerns  

Missouri S&T  

Wikia Technology  

Chemistry - Alkenes to Alkanes  

Lookchem - 1,3-BUTADINE  

Global Oneness - butylene  

Science Direct - Mortality study of workers employed in 1,3-butadiene  

INEOS Olefins & Polymers USA   

Storming Media    

Hazardous Substances Data Bank (HSDB^®)