While many people are welcoming the more spring-like weather, those with asthma are at a higher risk of suffering a life-threatening attack, a charity has warned.
Pollen is a major trigger for asthma attacks, with more pollen released on warmer, sunnier days.
Around one in five people in the UK are also affected by hay fever, which is caused by an allergy to pollen.
Hay fever and asthma are closely linked, with around 80% of people with asthma finding their asthma symptoms are triggered by pollen.
Hay fever from grass pollen in particular can be a risk to those with asthma, and research has shown that when there are higher concentrations of grass pollen in the air, more adults are admitted to hospital because of their asthma.
Sonia Munde, head of helpline and nurse manager at Asthma UK, said: "Pollen is a top trigger for asthma attacks at this time of year, affecting an estimated 3.3 million people in the UK with asthma.
"People with asthma who also have a pollen allergy not only experience classic hay fever symptoms such as itchy eyes and a running nose, but are also at an increased risk of a life-threatening asthma attack.
"Asthma UK's research has revealed that people with asthma say hay fever can disrupt their work, and has even caused teenagers taking exams to drop a grade."
She said those who have asthma and a pollen allergy should take hay fever medicines such as antihistamines and nasal steroid sprays, and make sure they take their preventer inhaler as prescribed.
These will help to relieve their symptoms and reduce their risk of an asthma attack.