Thank you for your inquiry.
I know of no guidelines which specify a temperature below which asthmatics should not go outside. It is true that cold air can cause asthmatics to wheeze, especially when they exercise, but most such episodes are transient and easily treated or prevented.
In general, most authorities encourage asthmatic children to pursue all normal activities in the same manner as children without asthma. Copied below are statements from both our national guideline (Expert Panel Report) and the Children’s Hospital of Pennsylvania which promulgate the concept that asthmatics should be encouraged to participate in any activity performed by children without asthma.
Thank you again for your inquiry and we hope this response is helpful to you.
"When should my child stop playing or come out of gym or a game because of asthma symptoms?
There should be no restrictions on children's ability to play, take gym class, or compete in sports just because they have asthma. If your child has been instructed to take medicine before physical activity, however, make sure that he or she does so every time. Some parents hesitate to give medicine before exercise or sports because their child seems fine or is active and doesn't develop symptoms." Source: Children’s Hospital of Pennsylvania Website
"Active participation in physical activities, exercise, and sports should be promoted." Source: Expert Panel Report 3 (EPR3): Guidelines for the Diagnosis and Management of Asthma
Phil Lieberman, M.D.