Many things, as normal pump fuel is a complex mix of over 50 different chemicals, several of which are unpleasant. However, the one I am most interested in is benzene.
Benzene is seriously nasty - the following is copied from the US government guidance on benzene but you can find similar from many sources including the World Health Organisation (ie this is based on proper research, not spurious opinion):
Immediate signs and symptoms of exposure to benzene
People who breathe in high levels of benzene may develop the following signs and symptoms within minutes to several hours:
Drowsiness
Dizziness
Rapid or irregular heartbeat
Headaches
Tremors
Confusion
Unconsciousness
Death (at very high levels)
Direct exposure of the eyes, skin, or lungs to benzene can cause tissue injury and irritation.
Long-term health effects of exposure to benzene
The major effect of benzene from long-term exposure is on the blood. (Long-term exposure means exposure of a year or more.) Benzene causes harmful effects on the bone marrow and can cause a decrease in red blood cells, leading to anemia. It can also cause excessive bleeding and can affect the immune system, increasing the chance for infection.
Some women who breathed high levels of benzene for many months had irregular menstrual periods and a decrease in the size of their ovaries. It is not known whether benzene exposure affects the developing fetus in pregnant women or fertility in men.
Animal studies have shown low birth weights, delayed bone formation, and bone marrow damage when pregnant animals breathed benzene.
The Department of Health and Human Services (DHHS) has determined that benzene causes cancer in humans. Long-term exposure to high levels of benzene in the air can cause leukemia, cancer of the blood-forming organs.
Normal pump fuel is approximately 2% benzene. Burned in an efficient car engine this is not too much of a problem - combustion is effectively complete so the emissions are negligible.
A 2-stroke engine (whilst having a much better power to weight ratio) is much less efficient, sometimes as low as 66%. The remainder comes out as unburnt fuel, which is about 0.6% benzene. To put that in context, from a 1litre tank you are putting 6ml of benzene into the air. Burn your way through 5l of fuel and that's 30ml. The permissible exposure limit in the US is 0.5 parts per million and the LD50 (rat), which is the dose at which half of rats to which it was administered died, is 1.8g/kg. If it scales, that would make the LD50 for humans in the region of 120ml, although that's for short term exposure rather than the long term insidious effects.
Obviously you won't breath all of the fumes in, but it's certainly something to think about.
Hope that helps
Alec