Symptoms of a pulmonary embolism include pain in the chest or upper back, difficulty breathing or coughing up blood.
The NHS website says people with these symptoms need to see a GP.
Someone with these symptoms may also have symptoms of DVT, such as pain, redness or swelling in the leg.
If someone has severe difficulty breathing, their heart is beating very fast or they have passed out, they need to go to A&E or call 999 immediately.
Full advice on blood clots can be found on the NHS website here.