Letters.

Bags under our nose

As a former director of engineering of a defense-products company, I'm very aware of the explosive nature of sodium azide ("A deadly threat in undeployed airbags," SN: 4/15/00, p. 251). I know that there have been serious explosions in industries Including the airbag industry) that use metal azides.

A few reasons sodium azide may have been selected for use in airbags are it releases gas fast, the gas it releases is pure nitrogen (of course, when sodium azide is mixed with other explosive or pyrotechnic ingredients, the resulting gas composition is altered), and it's fairly stable when it's inside an airbag system under most environmental conditions automobiles encounter.

Some reasons that airbag companies should be seriously looking for alternatives to sodium azide include the lethal threat to anyone exposed to sodium azide in the presence of moisture, which your story addressed quite well; the compound's explosive nature; and compatibility issues concerning the use of azides. In the event that a chemical pathway somehow leads to the formation of more-sensitive metal azides, the resulting chemicals are more prone to accidental detonation.

It's a tribute to airbag designers, airbag companies, and automobile manufacturers that they haven't had more problems with sodium azide.

James F. Kowalick Oregon House, Calif.

In reading this article, I didn't see any reference to what, if anything, is being done to regulate the disposal of airbags in light of the health hazard involved. Also, it's not clear to me what the effect will be on the general environment as we accumulate millions of airbags in junkyards around the world. I found myself terrified by this article.

Al Rosen Aptos, Calif.

The matter of the heart

Regarding the discovery of the dinosaur heart with the single aorta ("Telltale dino heart hints at warm blood," SN: 4/22/00, p. 260), your readers should note that this morphology is more likely to be related to high blood pressure than metabolic rate per se. The typical reptilian heart, with its incompletely divided ventricle and double aorta, is quite functional at separating oxygenated and deoxygenated blood. The reptilian heart also has the advantage of being able to shunt blood so that equal volumes needn't be sent to the lungs and the body.

However, with the higher blood pressure of birds and mammals, the lungs must be protected against high systemic pressures. This necessitates a completely divided heart and the accompanying loss of the accessory aorta. While high blood pressure is correlated with endothermy in birds and mammals, it would also be essential in a large animal with a long neck (as in many dinosaurs) for blood to be delivered to the brain, whatever the metabolism of the animal.

Christine Janis Brown University Providence, R.I.