In a view to a kill, James bond (Roger Moore) is knocked-out and pushed into a Silver Cloud Royals-Royce. Hoping to drown Bond, the villains push the car into a lake. As the pours into the car, he gains consciousness but instead of swimming up to the surface, where he will be seen, he stays under the water. He opens up a car-tyre valve and breathes the air bubbles saving his life! Is this really possible?

Let us do a rough 'back-of-an-envelope' type calculation to see if it's possible. To get a rough idea of the volume of a lung-full of air i.e. the volume of a good breath, we can blow into a balloon. We find it fills roughly halfway and so by filling balloons from an inflated car tyre we find that it contains the equivalent of roughly 30 half-balloons or breathes. But is this the same under the water?

When the valve is opened the air rushes out until the pressure inside the car tyre is the same as that outside. Atmospheric air pressure is the result of the weight of the air above us in the atmosphere. When we go under water the weight of water adds even more pressure. The weight of a column of the Earths atmosphere going straight up 50km or so, is about the same as a similar column of water just 10m deep.

If we go down deep enough the pressure of the water will eventually be about the same as that in the car tyre (ignoring how the tyre itself collapses under the water pressure). If we now open the valve no air will actually come out! If we assume that the car goes down just a few meters then the pressure won't be as great as this but even so less air will come out of the tyre than it would if it was on the surface. Let's say that we now have the equivalent of 20 or so of lung-fulls of air.

The air that comes out will be compressed by the water pressure but our lungs still require the same volume of air to breathe comfortably. So these 20 lung-fulls will probably reduce down still further to say 15 lung-fulls (almost half the available air we would have had opening up the valve on the surface).

In the film we see that much of the air is being lost bubbling away, if we say that half gets lost then that leaves him with about 7 lung-fulls. If one lung-full allows James to stay under water for say 30 seconds then we have a total of about 3 minuets of air. Of course he also has another 3 tyres on the car so theoretically, he could be down there for 10 minuets or so.

However, not all air is breathable. Every year people die when they dive down to explore sunken ships. They find trapped air in compartments and take off their masks thinking they will be able to breath. Unfortunately over time much of the oxygen gets used up rusting the ship's metal parts and what looks like breathable air may in fact be mostly nitrogen. Divers can easily suffocate because although the air is at the right pressure and feels comfortable to breath they actually get less and less oxygen in their lungs with each breath.

Car tyres are often topped up with air to keep the correct running pressure but there are iron reinforcement strips in tyres for example which will oxidize over time, so there may be less oxygen in the car tyre than expected depending on their age etc.

In our Hollywood Science TV series we tried it out. We could breathe in the bubbles when under the water and that the air tends to displace any water that might come in. If your head is down each gulp of air pushes any water to the front of the mouth and then you can swallow the air, we found we could stay under quite comfortably for as long as there was air - amazing!

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