Asthma and Lung UK said 3.1 million people could be at risk of a life-threatening asthma attack this week.
It comes as the Met Office has said pollen levels are set to be ‘very high’ across England and Wales from Wednesday until Friday, when the temperature is set to soar as high as 34C.
The Met Office said that “these rising temperatures also coincide with a rise in pollen levels which are expected to peak, especially across England and Wales, showing very high levels of pollen.
“Those vulnerable to high levels of pollen should be aware of the levels building through the week and take action to lessen their risk.”
Meanwhile, a study by Asthma and Lung UK has found that pollen was a trigger for 59 per cent of people with asthma.
The charity is advising people to carry their inhalers with them and check the pollen and air pollution forecasts this week.
Emma Rubach, head of health advice at Asthma and Lung UK, said: “More than three million people with asthma are affected by pollen and when pollen levels are at their highest this can be deadly for people with lung conditions like asthma.
“Hay fever can trigger asthma symptoms and increase the risk of a potentially life-threatening asthma attack.”
Rebecca Grandison’s seven-year-old son Wilfred has severe asthma, and said her son reacts so badly to pollen he ends up hospitalised regularly because of it.
The mum-of-two, 42, from Cheshire, said her “whole family revolves around the pollen forecast”.
She said: “When it is high, I can only let Wilfred play in the garden at midday as the pollen is higher in the sky then.
“This is the only time we are able to open the windows too.“
Ms Rubach said there are things people who have asthma can do to look after themselves.
She added: “Using your preventer inhalers every day as prescribed is really important as the medicine dampens down inflammation in the airways, helping to prevent symptoms such as wheezing and coughing before they even start.â¯
“We’d also advise you to carry your reliever inhaler with you every day, especially when you are out and about enjoying the sunshine in case pollen does trigger your asthma symptoms. Reliever inhalers quickly relax the muscles in the airways and ease symptoms on the spot.
“The third thing people can do is to use a steroid nasal spray every day, together with non-drowsy antihistamine tablets to help reduce the allergic reaction.
“People should also get into the habit of checking pollen and air pollution forecasts in their local area, so they can avoid going outdoors as much as possible on high pollen days.”