In an interview with the weekly Humo magazine in Belgium, and reported widely by the Flemish media, Boonen reveals one of his daughters uses an inhaler every day. He also reveals that he decided to retire after the spring Classics of 2017 so he could spend more time with his twin daughters. Never afraid to speak his mind, Boonen recently said Peter Sagan should "keep his mouth shut" over a lack of cooperation from rivals.
Froome's urine sample from an anti-doping control taken after stage 18 of the 2017 Vuelta a Espana was found to have twice the allowed limit of salbutamol. The 32-year-old says he is a life-long asthma sufferer, and insisted that he knows the rules and has never taken more than he is allowed.
UCI rules do not require a provisional suspension due to salbutamol being a specified substance but many, including UCI president David Lappartient, have expressed concerns over Froome competing while the case is still pending. Lapparitient has suggested it would be a disaster if Froome started the Tour de France sub-justice, but the Team Sky leader is determined to defend his name and target the Giro-Tour double.
Some riders have criticised Froome for his defiant stance but Boonen is more supportive.
"To call salbutamol doping is ridiculous," Boonen is quoted as saying in Humo.
"My daughter has to puff every day and something wrong can happen. I get the impression that Team Sky riders have already been declared guilty."
Team Sky is in the spotlight for the Froome salbutamol case, the infamous Jiffy bag delivery to Bradley Wiggins at the 2011 Criterium du Dauphine, his use of the banned corticoid triamcinolone via a UCI Therapeutic Use Exemption (TUE) and the series of allegations and contradictions that emerged during the British Parliamentary investigation.
Team Sky has been accused of ignoring their own self-professed ethics and using the grey areas of the sport to improve performance.
"So far, there is little hard evidence against Team Sky," Boonen said in Team Sky's defence.
"I'm really against cheats, they should be suspended for life, but you would be surprised to know how many athletes with respiratory problems need salbutamol to do their sport. Are we going to treat them all as EPO users?
Boonen retired a year ago, after finishing 13th at the 2017 Paris-Roubaix. He turned professional in 2001 with the US Postal Service team and went onto win the Tour of Flanders three times and Paris-Roubaix four times.
He occasionally works as a television commentator and is advising the Lotto Soudal team but explains he decided to retire to spend more time with his young family.
"Valentine and Jacqueline are the most important reason for me retiring. I want to be present as a father and when you are rider that's not easy," he said.
"I could have raced for another two years but I felt that they noticed my absence. I don't know if we'll have a third child. I always wanted four, but two at once keeps your feet on the ground. We are happy now and have two healthy children. Everyone knows our history [Boonen's wife Lore suffered two miscarriages – ed] and I don't want to defy fate."