Pulmonary emboli or blood clots in the lung; deadly? very dangerous?

Can be deadly: The size of the clot dictates how dangerous it is. Very very tiny emboli may not have much clinical consequence, unless it happens frequently. A large embolus that blocks large amounts of flow to the lung can cause symptoms including shortness of breath and even cardiac arrest. Unexpected shortness of breath should always be considered an emergency and evaluated by a doctor right away!

Yes. : Pulmonary embolism is often fatal but it really depends on the size of the clot that travels to the lung. Small clots may not even be noticed but large ones completely stop lung circulation and exchange of oxygen resulting in rapid death.