Mars attacks: How much dust from the red planet is safe to breathe?

NASA has released a report recommending an initial health standard for the Martian dust that astronauts could inhale during future missions. Although sending people to Mars is still years away, NASA needs to establish safety limits now. Spacecraft, spacesuits and crew habitats take years to design and test, and engineers need a measurable target for developing air filters, airlocks and suit-cleaning systems that will keep astronauts safe.