This year's heady bout of risk aversion on financial markets has ratcheted up demand for gold, U.S. Treasuries and the Swiss franc to levels that suggest they may no longer be the safe havens they are billed as.

Some investors see all three as vulnerable to a sharp sell-off should the global economic environment improve over the coming months, or simply because prices are too high in the absence of outright financial catastrophe.

A safe asset is something that is going to be safe across economic environments, said William De Vijlder, chief investment officer at BNP Paribas Investment Partners. It means you'd better make sure your forecast is right.

There are already signs the demand froth is coming off, at least in gold and the Swiss franc.

All three safe havens highlighted have distinct features, so losses from renewed demand for riskier assets would not hit each equally. Gold, for one, might fare better given that underlying demand for the metal is not all based on risk aversion.

But none are safe in all circumstances, and their remarkable rises this year may now pose some risk for those holding them.

Ten-year U.S. Treasuries recently traded with yields below 2 percent -- their lowest in generations -- and, according to Merrill Lynch data, have returned some 11 percent over the summer.

The Swiss franc has risen by 15.3 percent and 8.5 percent, respectively, to record highs against the dollar and euro, prompting moves from the Swiss National Bank to rein its currency in.

Perhaps most spectacularly, gold has risen as much as 33 percent, taking it to just below $2,000 an ounce -- a startling performance on top of a decade-long rally that has seen the metal's price rise more than 600 percent.

In the last few days, however, there has been a sharp sell-off -- nothing really to dent the asset's major gains, but a reminder of how quickly heady gains can run out of steam.

It is not difficult to believe that gold could correct a reasonably good amount, said Ashok Shah, chief investment officer at London & Capital, adding that this would not necessarily undermine its long-term bullish trend.

DIFFERENT RISKS

Of the three assets benefiting richly from the slowing global economy, lower interest rates and debt crises in the euro zone and elsewhere, gold is arguably the least vulnerable to a huge reversal.

It offers no yield or dividend and can rise and fall rapidly based on investor fear alone.

But the drivers behind its rise are diverse and it may hold up better than others when economic conditions change.

Demand has been bolstered by central banks buying the metal as part of their diversification of foreign reserves. Even more significant may be the buying of bars and coins by newly wealthy Asian consumers, notably those in China.

The World Gold Council estimates there was a roughly 25 percent rise in demand for gold from Chinese consumers between the second quarters of 2010 and 2011,

Gold is also not particularly subject to what BNP Paribas' De Vijlder calls the feedback loop, which occurs when a significant price rise begins to affect economies and prompts policy changes by governments.

The same cannot be said for the Swiss franc, which has also wobbled recently courtesy of the SNB's moves to cap its gains.

The SNB has cut official rates to near zero and pumped out more money to lower the franc's value. It has also sold francs in the forwards market to drive rates lower and make it expensive to hold the currency.

This is only a part of what it could do, meaning investors will have to battle to protect gains -- something that detracts strongly from the concept of a safe haven.

Charlie Morris, head of absolute returns at HSBC Global Asset Management, believes investors have been treating the Swiss franc as something that it is not.

It is easy to forget that the Swissie is a relatively minor currency and not the global liquidity pool that it is cracked up to be, he said.

'POINTLESS AND DANGEROUS'?

U.S. Treasuries, meanwhile, are at the point where investing in them is only slightly more lucrative than putting money under the mattress.

They are supported, like gold, by outside-the-market factors such as Federal Reserve buying and huge inflows from China. Some of that will change as the U.S. economy improves or as Beijing diversifies.

Mainly, though, yields of around 0.2 percent for short paper and only 2 percent for long, offer little. It would not take much of an inflationary spike or economic rebound to prompt a rush to the exit.

Treasuries are either pointless at the short end or dangerous at the long end, Morris said. Either we have deflation and bonds deliver paltry yields ... or, more likely, inflation resurges and investors in bonds lose their shirts.

(Additional reporting by Anirban Nag and Jan Harvey; Editing by John Stonestreet)

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