If a single grain of sand were to collide with your chest at 99.9999999999999999999999999999999999999999% of the speed of light, it would have an enormous amount of energy. The impact would be extremely destructive and could potentially be lethal.
When an object travels at such a high velocity, its kinetic energy increases significantly due to the relativistic effects described by the theory of relativity. The energy of an object in motion is given by the equation E = (γ - 1)mc², where γ is the Lorentz factor (which becomes extremely large at near-light speeds), m is the object's rest mass, and c is the speed of light. As the velocity approaches the speed of light, the kinetic energy becomes increasingly immense.
The specific effects of such a collision would depend on various factors, including the mass and size of the grain of sand, the specific properties of the material it is composed of, and the exact angle and location of the impact on your chest. However, it is safe to say that a collision at such an incredibly high velocity would likely result in severe damage or even fatality.
It's worth noting that this scenario is purely hypothetical since it involves an object moving at a speed that is extremely close to the speed of light, which is currently beyond our technological capabilities to achieve.