CHARLES DICKENS' A Christmas Carol is a tale of ghosts. There are the ghosts who visit Ebenezer Scrooge and show him Christmas past, present and future. Then there are the people who were as invisible as ghosts to men like Scrooge: London's poorest of the poor, starving and cold in the midst of plenty.

These hard and cruel times did not end in the 19th century. In Philadelphia today, we have neighborhoods teeming with the poor - men, women and children as destitute as London's poor in Dickens' time. One in five Philadelphians live below the poverty line.

In Dickens' time, the poor had them three options: private charity, the poor house or starvation. Government gave little aid.

Today, government does help - with programs such as food stamps, subsidized housing and temporary financial help. The safety net exists - at least for now. But the poor live on the edge.

Consider something as simple as shelter. Thanks to the federal government, the Philadelphia Housing Authority provides subsidized housing to 81,000 people - some in PHA housing, others using Section 8, the voucher program that subsidizes rents in privately owned homes and apartments.

Still, demand exceeds supply. There are nearly 43,000 people on PHA's waiting lists, all of whom are poor enough to qualify. A step above, there are thousands - many of them the working poor - who live in substandard housing, paying rents take a huge bite out of their income.

As reporters Barbara Laker and Wendy Ruderman recently disclosed in a Daily News/Inquirer series, a lot of those dwellings have intolerably high levels of lead, a substance that is toxic at high levels. We like to think of lead poisoning as a disease of the past - like polio and typhoid. But, it lives on - thanks to landlords unwilling to clean up the sources of lead and a city that fails to enforce existing laws.

A recent Federal Reserve Bank study showed more than one-third of all Philadelphia renters are classified as "severely cost burdened," which means they spend at least 50 percent of their income on rent and utilities.

The same study showed a decline in lower-cost rentals. According to the researchers, while the number of rental units over $750 a month have increased by 57,000 in the most recent five-year period, the number of low-income units has gone down by 24,000. (The study defines low-income units as below $750 a month.)

Some of this is due to gentrification: Neighborhoods that see big jumps in home values also see big increases in rent that can be charged. Some has to do with the law of supply and demand: Despite the increase in the number of rental units, landlords can and do charge more.

Low income people get left behind when rents rise, and the Federal Reserve study shows the trend is not going to be reversed. Finding affordable housing is going to become a greater challenge.

In Dickens' London, poverty and prosperity lived side by side. Modern Philadelphia has the same stark contrast between wealth and want. The Federal Reserve study tells us there is a growing need for affordable, decent housing. It is a problem we need to address - knowing that while those affected might be invisible to the rest of us, they are not ghosts. They are real people with a real problem.