Go through these in order and stop at the first one that applies. If you are reading this for someone else, answer for them.
1
Is any of this happening now?
Call 911
Call emergency services if breathing is severe, began suddenly, or is getting worse quickly.
Emergency help is also needed if breathlessness occurs with:
Chest pain, pressure, or tightness
Blue, grey, or unusually pale lips or skin
Fainting, confusion, extreme drowsiness, or difficulty staying awake
Inability to speak in full sentences
Choking or swelling of the lips, tongue, or throat
Coughing up blood, especially with chest pain or a swollen leg
And for anyone, someone who:
is struggling to breathe, gasping, choking, or cannot get words out
has lips or skin that look pale, blue, grey or blotchy
Other emergency signs, for anyone
Someone who has chest pain, tightness or pressure lasting more than a few minutes
Someone who has sudden weakness or drooping on one side of the face or body, slurred speech, or sudden confusion
Someone who is unconscious, or cannot be woken
Someone who is bleeding heavily and it will not stop with firm pressure
Someone who has had a seizure that will not stop, or does not wake up afterwards
Someone who you believe has taken an overdose or swallowed something poisonous
2
Is any of this happening?
Be seen today
Seek same-day medical advice for new breathlessness at rest, a clear worsening of an existing breathing problem, or breathlessness with fever, wheezing, or a fast or irregular heartbeat.
You should also seek prompt care if you are pregnant, recently had surgery, or have been immobile and develop unexplained breathlessness.
Gradual breathlessness is still worth discussing, even if it does not feel urgent.
And for anyone, someone who:
is getting rapidly more short of breath, or has gained several pounds in a few days with swelling
3
Does any of this apply?
Book an appointment
Book a routine appointment if the symptom keeps returning, lasts longer than expected after an illness, or limits walking, stairs, sleep, work, or other usual activities.
And for anyone, someone who:
is more tired or breathless than usual doing things they used to manage
The article also says
Do not drive yourself if you are severely short of breath.
Every sign above is quoted from the sofidex article on Shortness of breath. If you are not sure, err upward — it is always reasonable to be checked.