Roslan Rahman | AFP | Getty Images

For one month every year, Singapore takes a hiatus from its reputation as a squeaky clean city to feed the hungry ghosts during the annual Chinese festival. During Hungry Ghost Month, the seventh month on the Chinese lunar calendar, the gates of hell open, allowing the ancestors' ghosts wander the earth, according to Chinese tradition. The ghosts are either honored or appeased -- depending upon their character -- with offerings of food, incense and the burning of paper effigies of money or other gifts. Singapore may present a modern, often Western-appearing, facade of gleaming office buildings and high-rise apartment buildings, but it hasn't let go of the tradition.

Offerings left at a public housing estate in Singapore Leslie Shaffer

Its holiday offerings add up to a substantial sum. The city-state imported more than 21 million Singapore dollars-worth ($16.8 million) of joss paper from January to June this year as well as nearly 3 million Singapore dollars-worth of religious scented powders and incense, according to data from International Enterprise Singapore, a government agency.

The city-state -- the only country considered a developed market in Southeast Asia -- backs up its reputation for cleanliness with fines as high as $10,000 Singapore dollars for littering, but when it comes to the ancestors, Singaporeans give the rules a wink. Many eschew the specialized bins set out on public housing estates for burning joss paper, leaving the remains of their offerings at various locations throughout parks and residential areas.

Stepping on or cleaning up the offerings could anger spirits, who may follow the offender home, according to tradition. Read More Chinese can't say neigh to feng shui in year of the horse

To be sure, Singaporeans may get messy with their burning, but they don't appear to get careless. The Civil Defence Force, which takes responsibility for fighting fires, noted that there's no uptrend in incidents it responds to during the holiday period and in fact, the total number of fires it responds to has been trending downward for years. It's difficult to tease out whether the burning during Hungry Ghost Month causes more hospital or doctor visits. The holiday period coincides with the "haze season," when neighboring countries clear rain forests by burning, sending smoke wafting over the city-state. Data from one hospital, Changi General, suggests there isn't much difference in the number of respiratory cases there before and during the holiday period. But some patients may avoid the hospitals out of concern that spirits could be more active there.