"He has a chronic lung condition that could worsen at any time," Ambassador Ana Alban said on Assange, who is fighting UK efforts to extradite him to Sweden to face allegations of rape and sexual assault.
"Mr Assange, as everyone knows, is in a confined space," Alban told reporters in the Ecuadorian capital ahead of an annual gathering of the country's top diplomats with President Rafael Correa.
"Not only does the embassy have few windows but the city is also dark at this time - we have very little daylight in London. He is exposed to any health consequences from this lack of sunlight and lack of fresh air," Alban was quoted as saying by the BBC.
Ecuador previously sought assurances he would not be arrested if hospitalised.
The UK said then it would not prevent "any medical care that he requires".
Assange, 41, an Australian national, had taken refuge at the embassy on June 19. He was granted asylum by Ecuador in August.
Assange, who has breached bail conditions by staying at Ecuador's embassy here, faces arrest if he leaves.
He is wanted for questioning in Sweden over rape and sexual assault allegations, which he denies. He was arrested in the UK on an extradition warrant.
She said Ecuador was "waiting for a meeting" with either UK Foreign Secretary William Hague or UK Home Secretary Theresa May to discuss Assange's future.
Since 2010, Assange's WikiLeaks website has published hundreds of thousands of leaked US diplomatic cables, angering Washington.
In 2010, two women accused Assange of committing sexual offences against them while he was in Stockholm to give a lecture. Assange says the sex was consensual and that he believes the case to be politically motivated.
He says he fears being passed on to authorities in the US if extradited to Sweden.
But Swedish prosecutors have dismissed Assange's claims their case is part of a wider political move to see him stand trial in the US over his work with Wikileaks.
