The capital has been placed on a Level 6 warning compared to it's usual Level 2 rating and Asthma UK have said those with breathing problems are most vulnerable.
Director of Advice Andrew Proctor said: "People with asthma need to take extra care.
"We are facing a perfect storm of asthma triggers, with air pollution and weather changes increasing the risk of potentially fatal asthma attacks for millions of people with the condition.
"Two thirds of people with asthma find that air pollution makes their asthma worse and even more tell us that changes in the winter weather can trigger asthma attacks."
Tips to manage asthma
Carry your inhaler with you at all times - 1 in 11 people in the UK have asthma and many attacks are unexpected, so it is always worth carrying them with you
Check weather and pollution forecasts for your area. If you're a smartphone user, there are even apps which tell Londoners when to avoid toxic fumes and where the worst spots are
Make sure you have enough medication to see you through the Christmas and new year period, when pharmacy times are different
Be equipped and know what to do if you have an asthma attack. It is recommended by GP's that you must stay calm and sit up straight, take a puff of your inhaler every 30 seconds up to 10 puffs if necessary and make sure someone calls 999
Know your triggers. Booking regular appointments with your asthma nurse can help identify what your triggers are, so you can manage those better.
Make sure your medication is somewhere that is easy to reach, particularly when travelling around London. In a busy tube, it is important you can reach your inhaler if necessary