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Name: Rachel
Status: student
Grade: other
Location: VA
Country: USA
Date: Fall 2009

Ozone is a poisonous gas the can build up in the air in dense cites. Thus, there are many environmental initivatives to lower the amount of ozone in the air. One way you can make ozone, however, is by baking bread. The nice smell that you associtae with baking bread is actually due, in a part, to ozone. If ozone is poisonous, why is baking bread not considered a dangerous activity? I have to challenge some of your premises. First: Where is the delightful odor of freshly baked bread shown to be due to ozone? Where is the data? I don't think there is a connection there.

I have to challenge some of your premises. First: Where is the delightful odor of freshly baked bread shown to be due to ozone? Where is the data? I don't think there is a connection there. Second: This is more important. A given chemical substance -- it could be that ozone, or any one of thousands of other chemical substances that can be toxic in one environment, but beneficial in another. Ozone is an excellent illustration. Along an urban highway, ozone is irritating and toxic, but in the upper atmosphere, it is a very important substance that blocks ultraviolet radiation from reaching the surface of the Earth.

You do raise an important point. The "toxicity" of a substance has to be measured in the entire context of the substance, not just its "intrinsic" toxicity, but also the amount, the presence of other substances, as well as the pathway of exposure. A "classic" example (although I wouldn't test the experiment) is rattlesnake venom. It is reported to be non-toxic if swallowed, but obviously toxic inserted by the fangs of the reptile.

The point is that "toxicity" is a complicated interaction of a substance and other factors. In one context a substance may be toxic, and in another context the same substance may be quite harmless.

Vince Calder

Although ozone is poisonous, it requires a high concentration to be dangerous. The amount released in baking too small to be dangerous.

Not only is ozone associated with baking, but is that "fresh" smell we get after a thunder storm is ozone that the lightning has created.

Other gases are poisonous in higher concentrations including oxygen!

R. W. "Bob" Avakian
B.S. Earth Sciences; M.S. Geophysics
Oklahoma State Univ. Inst. of Technology


Thanks for this interesting question, this isn't the first time I've heard this kind of thing come up. Ozone is a highly reactive gas normally formed in the upper atmosphere. As a chemist I've used ozone to carry out chemical reactions such as oxidative cleavage of organic molecules, but it requires extreme conditions to form it. In the lab we use high voltages acting on normal oxygen gas to form it, in nature it is usually formed by the action of ultraviolet light or lightening strikes on oxygen. I would not expect it to form in natural, fairly ambient conditions such as baking bread.

However, there is a grain of truth in the idea that ozone is formed from baking bread. The process of fermentation of sugars in bread produces carbon dioxide (that makes the bread rise) and ethanol (alcohol). The amount of alcohol formed is relatively small but the conditions in the oven cause it to be lost as alcohol vapour. This is part of the aroma that you smell when bread bakes. This poses no real health risk but large bakeries that produce a substantial amount of ethanol vapour can cause local air pollution by the action of sunlight on the mixture of air and the ethanol vapour in the vicinity. This reaction is similar to the photochemical smog that forms from unburnt hydrocarbon emissions around cars etc. One of the possible products of these types of reaction is ozone, a secondary pollutant.

Best wishes,

Tom Collins

You have to weigh the benefit of the bread produced against the danger of the small amount of ozone produced. People aren't very good as assessing relative risks and benefits, but it's worth trying to do this well, so that we put our effort where it will do the most good. This is one place where science really comes in handy. --

Tim Mooney

Hi Rachel,

The key to keep in mind is the amount of the substance, also known as the "dose" or "exposure". The hazards of exposure to a substance depend on how much of it to which you're exposed. Some things are only harmful in very high doses, while other things can be harmful at very low doses. For virtually any substance, there is a lower limit where the substance is not harmful.

I'm not familiar with the ozone levels in bread, but I'm fairly confident they're well below the exposure limit / harmful dose for ozone.

For many substances, a government agency called OSHA (Occupational Health and Safety Administration) publishes exposure limits for workers. These are based on a single exposure or routine exposure. For more information about exposure limits, here is a link to the OSHA web site: Similarly, the US EPA (Environmental Protection Agency) puts limits on how much of various hazardous substances can be in drinking water yet still be safe to drink.

Hope this helps,


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