The

Global Portfolio

mailbag has been piling up again. Time to dip in and answer some questions -- and respond to some broadsides.

My column on

green investing generated a great number of responses and questions. While some of the comments were positive and others negative, all revealed a strong interest in investing in companies trying to clean up the earth.

One reader took issue with several points in the column, beginning with my statement: "Take fuel cells, which are destined to replace gasoline as the energy driving automobiles in the next decade."

The reader points out that "fuel cells are energy converters (like an engine), not energy sources (like gasoline). FC systems might replace gasoline engines, but the FCs will need a fuel to provide energy. Several developers have been working on fuel reformers that would let FCs operate on gasoline."

In addition, he says, "even the most optimistic proponents of FC vehicles expect them to account for only a small minority of automobile sales 10 years from now -- 'destined' is a word that reflects far greater certainty than the facts merit." This reader is concerned because:

When FCs became a hot investment earlier this year, the interest was accompanied by a flood of misinformation from touts and shady companies. The touting, together with investor ignorance, contributes to the volatility the FC stocks have seen, harms investors, enriches shady characters and tarnishes the image of FC companies in the financial markets.

First, a

mea culpa

. To be more accurate, my column should have said, "fuel cells are destined to replace current internal combustion engines powering automobiles." Fuel cells have been or are being developed to use a variety of fuels, including gasoline. Hydrogen and methanol are the cleanest, but even a gasoline-powered fuel cell system would reduce greenhouse emissions by 20%. It is still unclear which fuel will prevail.

However, the scientists and investors I spoke to while researching the article are confident that fuel cells will become more important over the next 10 years. Does that mean everyone will be driving a car with fuel cells a decade from now? No, but I do believe we will be heading that way and using fuel cells to power appliances, cell phones and other items. Over the long term, the companies that master that technology will be good investments. There is, after all, volatility everywhere these days.

Having said that, I completely agree with your concerns on information about companies developing fuel cell or other environmental technologies. Many of those companies, even if they are behaving ethically, are young and without track records. They are developing technologies that are enormously complex. Their products may or may not catch on, and therefore their future earnings may be difficult to predict. (See the

H2FC Web site for a good source of information about investing in fuel cell companies.) Although environmental technology has great potential, investors should approach the sector with caution. I am always surprised and a bit dismayed by the number of readers who have questions about companies they have already invested in.

Meanwhile, one of the companies mentioned in the piece, fuel cell developer

Plug Power

(PLUG) - Get Report

, suffered a setback last week when it announced that changes to fuel cell technology relieved

General Electric

(GE) - Get Report

of the obligation to buy fuel cells under a 1999 contract. Plug's stock had tumbled 38.5% as of Tuesday.

Merrill Lynch

,

Bear Stearns

and

Goldman Sachs

all downgraded the company.

Ballard

(BLDP) - Get Report

, another fuel cell company, has fared better. It had risen 21% by late April but its share price has since returned to roughly where it was when I wrote the piece.

Elsewhere in the mailbag, a number of columns on Asia struck a nerve.

Jaafar

in Malaysia took me to task for

praising former Malaysian Finance Minister Anwar Ibrahim. Anwar "presided over the biggest 'boom and bust' cycle the country has ever seen," says Jaafar. A valid point, but however one rates Anwar's tenure, it certainly did not warrant imprisonment and torture. Meanwhile, investors seemed to have cooled on the country, as my column cautioned, with the climb in Malaysian stocks slowing prior to the reweighting of the country by

Morgan Stanley Capital International

in a key index this month. The benchmark

Kuala Lumpur Stock Exchange Composite Index

has dipped 2% since that column was published.

The two main vehicles for U.S. investors to play Malaysia have dipped as well.

WEBS-Malaysia

(EWM) - Get Report

has slipped 5% since the column appeared, while the closed-end

Malaysia Fund

( MF) is down 11%.

On my

piece on China's threats to Taiwan,

Michael

asked, "What happens when China finally makes good on the threat? You might see a prolonged situation on the island with the U.S. getting involved. Sooner or later you know it's going to happen, dude."

Perhaps, but no matter how hot the rhetoric gets, an armed conflict makes no sense for either side for the time being. In the meantime, savvy investors will continue to take advantage of cross-straits tensions. Since the piece appeared, China has refrained from more saber-rattling -- and the market has staged a modest rally, rising 2%.

Finally,

B. Milton Choate

did not like my

commentary on Vietnam, saying I was full of "hot air" and suggesting "it is paramount that you quit all of your present activities and go to Vietnam as an unpaid goodwill ambassador."

See, more proof the Cold War isn't over. Thanks, I'll pass. I'm no Jane Fonda.

Keep reading -- and writing.

David Kurapka's Global Portfolio column appears Wednesdays and Fridays on TSC. In keeping with TSC's editorial policy, he does not own shares in any companies or mutual funds mentioned in this column. He also doesn't invest in hedge funds or other private investment partnerships. He welcomes your feedback at

dkurapka@thestreet.com.