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
Seek emergency help now after a suspected mercury exposure if you or someone else has:
Trouble breathing, severe chest discomfort, or blue or grey lips
Collapse, a seizure, extreme drowsiness, or severe confusion
Repeated vomiting, severe abdominal pain, or bloody diarrhea
Sudden severe weakness, inability to walk, or major loss of coordination
Swallowed a mercury compound or an unknown mercury-containing product
Contact your local poison centre or emergency service for immediate instructions.
And for anyone, someone who:
you believe has taken an overdose or swallowed something poisonous
Other emergency signs, for anyone
Someone who is struggling to breathe, gasping, choking, or cannot get words out
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 has lips or skin that look pale, blue, grey or blotchy
2
Is any of this happening?
Be seen today
Get prompt medical or poison-centre advice after a significant spill in a heated, enclosed, or poorly ventilated space, even if no symptoms have appeared.
Also seek prompt advice about exposure during pregnancy or in a young child.
3
Does any of this apply?
Book an appointment
Arrange a routine medical appointment if you have possible ongoing exposure with gradual symptoms such as tremor, numbness, memory changes, or gum irritation.
Bring information about your work, diet, products, or spill if available, but do not bring contaminated material into a clinic.
The article also says
Move away from fumes if you can do so without entering a contaminated area.
Do not induce vomiting.
Every sign above is quoted from the sofidex article on Mercury poisoning. If you are not sure, err upward — it is always reasonable to be checked.