It was expected to make landfall again in South Carolina on Thursday, according to the Miami-based National Hurricane Center (NHC).

On Sunday, Ernesto had strengthened into the first Atlantic hurricane of the year, causing one death in Haiti before losing its punch Monday as it moved over mountainous areas of Cuba.

Experts initially feared Ernesto would hit Florida as a powerful hurricane packing the same intensity as Katrina, which killed 1,500 people after it slammed ashore near New Orleans, Louisiana, on August 29, 2005.

The initial concern prompted Florida Governor
Jeb Bush to declare a state of emergency, while visitors were ordered to leave the Florida Keys.

NASA canceled the Tuesday blast-off of the Space Shuttle Atlantis from Florida's Atlantic coast but changed its mind about moving the shuttle to its hangar for protection as the storm outlook improved.

Hurricane conditions are expected within 24-hours along a stretch of coastline northwest of Acapulco that includes the resort of Manzanillo -- popular with North American sailfish fisherman -- and the steel port of Lazaro Cardenas.

Category 3 hurricanes can create sea surges of 9 to 12 feet above normal, cause structural damage to small homes, blow down large trees and destroy mobile homes.

A storm warning was posted for an area covering Acapulco, although the hurricane was not expected to make a direct hit on the resort city where some 1 million people live.

"Some additional strengthening is forecast during the next 24 hours and John could become a category four hurricane," The Miami-based National Hurricane Center said.

Rainfall of 2 to 4 inches and isolated deluges of 8 inches were possible along the coast.

"These amounts could cause life-threatening flash floods and mudslides over areas of mountainous terrain," the hurricane center said.

THREATENING SKIES

Heavy sea swells and rain pounded Acapulco.

Dozens of coastal towns and cities were told to keep on alert, according to Mexico's civil protection agency. Ships and boats were warned to take extreme precautions.

"John is intensifying rapidly," the agency said in an early-warning note on its Web site.

The storm formed on Monday and quickly picked up power to become a Category 1 hurricane early on Tuesday before being upgraded two notches inside a few hours.

Nadya Velas, spokeswoman for civil protection in Guerrero state, home to Acapulco, said there were no plans for the moment to evacuate coastal fishing towns or mountain villages under threat from floods or mudslides.

"But since we heard it had become a hurricane we have stepped up the monitoring," Velas said.

John is expected to pound Acapulco with wind and rain and then batter other coastal resorts like Manzanillo, Puerto Vallarta and Los Cabos in the Baja California peninsula in coming days.

The port of Salina Cruz, which exports crude oil to Asia, was open but on alert, with the port authority reporting wind and choppy seas.

State oil monopoly Pemex also has a refinery at Salina Cruz, southeast of Acapulco, with refining capacity of 330,000 barrels per day. The hurricane was moving further away from the oil port.

In the new study, scientists found that three widely used, FDA-approved UV filters (octylmethoxycinnamate, benzophenone-3 and octocrylene) actually generate ROS in skin when exposed to ultraviolet radiation. So the sun's damaging effect is multiplied when the sunscreen has been on too long. [Graphic]

"Sunscreens do an excellent job protecting against sunburn when used correctly," said chemist Kerry Hanson of the University of California, Riverside. "This means using a sunscreen with a high sun protection factor [SPF] and applying it uniformly on the skin. Our data show, however, that if coverage at the skin surface is low, the UV filters in sunscreens that have penetrated into the epidermis can potentially do more harm than good."

Hanson and colleagues applied sunscreen to model skin tissue and imaged its travels into deeper layers and the effects.

"More advanced sunscreens that ensure that the UV-filters stay on the skin surface are needed," Hanson said.

Also, the researchers note that while most sunscreens block one wavelength-band of ultraviolet radiation, called UVB, few block the deeper-penetrating UVA. The federal government just approved in July a UVA blocker for use in the United States.

Meanwhile, the scientists have passed along the advice of the Skin Cancer Foundation, which recommends reapplying sunscreen every two hours, and especially after sweating or swimming.

The study, announced today and funded by the National Science Foundation and the National Institutes of Health, will be detailed in the journal Free Radical Biology & Medicine.