TORONTO - The spring thaw is being blamed for a foul, stinky smell that had residents in the Greater Toronto Area holding their noses this morning.

The stench was a hot topic on Twitter as people from Oakville to Whitby to Richmond Hill to Aurora weighed in on the topic.

One person tweeted the misspelled warning "Everyone plug there nose TO's BO is back" while another wrote "Dear #Toronto, why do you smell like a litter box?"

It was the second time in two weeks a nasty smell had wafted over the city.

Several people likened the smell to everyone stepping in dog poop while other descriptions of the nasty odour included garbage, sewage and rotten milk.

But Kate Jordan with Ontario's Ministry of the Environment said it's simply the smell of spring in the air as the ground thaws.

"The manure smell or the organic type smell ... that is common to this time of year when we see the temperatures warming up, the ground's getting softer, leaves are decomposing," said Jordan.

Mayor Rob Ford's office said the city was looking into the cause.

Toronto's water department investigated its systems and "everything's operating normally," said spokeswoman Cheryl San Juan.

"There's nothing within our systems that would indicate the odour's coming from us," she said.

The department notified the Ministry of the Environment's spills hotline about the odour because there were a number of complaints, she said.

To ensure there's not a specific source and public health is protected, environmental officers were out doing a "sniff test" in several areas where the smell had been reported, said Jordan.

"Given the kind of area that's complained, and the type of smells they're reporting it wouldn't be uncommon to be attributed to the spring thaw," she said.