This 'n That
There is no doubt that some woods can be potentially toxic to some people. The question is always one of "what degree". This cannot truly be answered. Each individual has different degrees of resistance, some more prone to allergic reactions, and others not so. Every day our knowledge of tree biology and chemistry grows, leaving us with more information, but also more questions. How all this relates to each individual is impossible to know. All we can say is "be cautious". Use new woods in a limited way, with proper respirators, until you know that it has no adverse reaction with your body.
Toxicity can mean a lot of different things to different people so here are some basic ideas:
Natural Chemical Poisons:
This normally refers to chemicals produced by the tree. It has been suggested that these chemicals originated as part of the tree's natural defense system against insect and other animal attack. There are higher concentrations in the sap, bark and foliage of the tree, much less in the trunk. For example, the foliage of black cherry contains a precursor of cyanide, but yet we work with the wood with little difficulty. Woods that are naturally oily, even after kiln drying are much more likely to cause an allergic reaction, just because the wood's oil can transfer onto your skin more easily and enter the blood stream. The reaction can range from that of a mild irritant, to something that is truly toxic and lethal, although the latter is very rare.
Natural Physical Poisons:
Fine wood dusts, even if they have no chemical toxicity, can pose a health risk. Dust can act as an irritant to the skin and particularly to the respiratory tract of people that have a natural sensitivity in this area. Western Red Cedar has an extremely small dust particle size, prone to penetrate deeply into the lungs and cause severe reactions in asthma sensitive people. Wear a dust mask and have good dust extraction equipment.
Nature can introduce toxins as the wood rots on the forest floor. Fungal spores and bacteria that invade a rotting tree, creating beautiful spalted wood, but can be toxic if they are able to invade the human body. Every time the wood is chemically treated with products like creosote to prevent rotting. Formaldehyde glues are used extensively to glue plywood layers together in both sheet materials and flooring. In sawing these materials, the dust generated introduces these toxins back into the air. Many of these glues emit vapors long after the manufacturing process has been completed. Both the dust and the vapors can be very toxic.
The Last Word...
Be cautious...just because something doesn't cause a reaction the first time doesn't mean you'll never have an allergic reaction. Your sensitivities can build with exposure.