WEDNESDAY, May 7, 2014 (HealthDay News) -- E-cigarettes may not be as harmless as they initially seemed. New research suggests that e-cigarette vapor produces tiny particles that users suck deep into their lungs, potentially causing or worsening respiratory diseases.

The particles are of comparable size to those contained in cigarette smoke, and as many as 40 percent of them reach the deepest part of the lungs when inhaled, said Jonathan Thornburg, lead investigator and a senior research engineer at RTI International, a North Carolina research institute.

That means if the particles turn out to be harmful, they'll be causing damage throughout the lungs.

"These small particles have a high surface area-to-volume ratio," Thornburg said. "When they deposit in your lungs, it makes it easy for whatever chemicals are in them to dissolve into your lung tissue." Those chemicals potentially could cause or worsen respiratory problems such as asthma or bronchitis.

In its review of emissions from two types of e-cigarettes, Thornburg's team did not find any toxic substances in the vapor produced by the devices.

"Everything we found was what the [U.S. Food and Drug Administration] and others generally regard as safe," he said, noting that the cancer-causing agents produced by burning tobacco are not present in e-cigarettes.

But another new study raises the possibility that the liquids used to produce e-cigarette vapors could contain carcinogens or harmful ingredients, The New York Times reports.

The study found formaldehyde, a known carcinogen, in overheated vapor produced by high-power e-cigarette devices known as tank systems, the newspaper reported. These systems are larger devices than typical e-cigarettes, and are designed to vaporize liquid nicotine quickly to give users a bigger nicotine kick.

These studies provide even more impetus for the FDA's recent proposal to begin regulating e-cigarettes as tobacco products, said Dr. Norman Edelman, senior medical advisor for the American Lung Association.

"We certainly don't believe e-cigarettes are a safe alternative," Edelman said. "The question is whether it's a safer alternative, and we believe those results aren't in yet. This is a tobacco product and should be regulated by the FDA as all tobacco products should."