I've heard that cooking with olive oil is bad for you and can be toxic.  Is this true?  If so, to what extent?  I see no warning on the container of my olive oil that states that it would be bad to cook with olive oil. 
I think this is a problem with all oils. When something burns, it produces smoke. Smoke is generally indicative of something that isn't particularly good for us if inhaled. Same holds for oils. It seems that all oils will begin to produce toxins once they hit their smoke point. However, before that, they are completely fine. So pick an oil that will handle the temperature you're using by checking the smoke point.   There's also a difference between producing toxins and toxic. Does burning oil produce toxins? Yes. Will that actually make you sick? I don't know. But my inclination is that it's unlikely to cause you harm under "normal" usage or else we'd be hearing about people actually getting sick from burning their oil rather than just getting scare stories on the news about free radicals.   And to get very sciency (stop reading immediately if your eyes glaze over, I think that's a sign of toxicity), from biology online:     Researchers investigated processes of   oxidative degradation - notably that   caused at 70°C with ventilation - of a   broad group of oils with very   wide-ranging compositions. Another   degradation process studied was that   which is caused by microwave action   that does not heat greater than a   temperature of 190°C.      In both processes deterioration of the   oils takes place. In the first type of   process (70°C with ventilation)   hydroperoxides are first produced and   subsequently aldehydes. In the second   kind of process (microwave) it is   basically aldehydes produced. It has   to be pointed out that both the   oxidative conditions and the   composition of the oil determined the   velocity of the degradation and both   the nature and concentration of the   compounds produced.       These studies have shown, for the   first time, that degradation of lipids   in foods can produce toxic oxygenated   aldehydes. These compounds, well-known   in medical studies for their geno- and   cytotoxic activity, considered as   markers of oxidative stress in cells   as well as being causal agents of   degenerative illnesses, had not   previously been detected in   foodstuffs.       Researchers have shown that some oils   produce these toxic substances in   greater quantities and at a greater   rate. Virgin olive oil was, amongst   all the oils studied, that which took   longer to produce this type of   compounds and produced a lower   concentration of them.  