Arthur Rozumek couldn't believe it when a tiled mural at Downsview Station was covered by an advertisement.

"It's a spectacular work of art, it makes the station a lot more distinctive, a lot nicer," said Rozumek, a law student, in an interview Thursday.

In fact, Rozumek was so concerned he wrote to CBC Toronto about it, saying the TTC was being "disrespectful to commuters and taxpayers," and that putting advertising on a work of art is "unacceptable for a public agency."

He often travels through Downsview Station and is fascinated by the large mural, made up of different coloured tiles of varying sizes.

A large piece of the artwork wraps around an escalator entrance, with the tile artwork continuing through other parts of the station along the lower level.

Commuter Arthur Rozumek can't believe the TTC would put an ad on top of artwork it commissioned. (Lorenda Reddekopp/CBC )

"I think it's important to note the taxpayers paid for this art. It's supposed to be for everyone's enjoyment, so we shouldn't be covering it up," he said

Arlene Stamp created the expansive mural, called Sliding Pi.

She was living in Toronto back in 1993 when she came up with the concept. She's now 78, retired and living on Vancouver Island.

Stamp taught math before becoming an artist. This project brings together those two passions.The different-sized tiles are based on the mathematical number pi and its non-repeating pattern of digits.

So how did she react when she learned it's now partly obscured by advertising?

"The effect in the skylight, which is my favourite part, has been interrupted, so I'm sad about that but I certainly don't feel precious about it."

Artist Arlene Stamp feels the sense of movement her artwork evokes is appropriate for a subway station and says that movement is "interrupted" by the advertisement. But she says she is not upset about the TTC's decision.

After seeing a photo, Stamp said she doesn't have much of a problem with the ad.

She remembers being paid $5,000 for her creation, which took many hours of work.

The large vinyl ad is for the School of Design at George Brown College.

Here's a closer look at the George Brown College ad on top of the artwork. (Lorenda Reddekopp, CBC News)

Stamp said it could be worse: "I'm really glad it's for George Brown College and not for Campbell's soup or something," she laughed.

This is far from the first advertisement plastered on that space.

Brad Ross, the TTC's executive director of communications, confirmed it's been used for years. It even at one time had an advertisement for the CBC-TV program Republic of Doyle.

Ross said it's tricky balancing act for the transit authority. It has a $61 million budget deficit for next year and advertising brings in $25-30 million a year, Ross told CBC Toronto.

"There are certain pieces of art that we couldn't or wouldn't cover but there are others that are more conducive to allowing for it temporarily."

Amrit Sohi is a George Brown student who travels through the station every day.

He has no problem with the school advertising in the subway station. He transferred into a construction management program from Ryerson University.

"Ads like this helped me out," said Sohi. "I saw an ad like this, went online, did some research and here I am at George Brown."

But University of Toronto student Mehran Shamit disagrees.

She didn't initially realize she was standing in front of a work of art, assuming it was just coloured tiles.

However, once she learned the background of Stamp's piece, she said: "I definitely don't think it's okay... There are billboards out there to put ads up but we definitely don't need to cover up artwork"