Lung cancer rates have declined 12 percent in the last three decades, according to a new study released by the National Cancer Institute. The study revealed several confusion issues, despite the encouraging picture yielded of the diminishing rates.

Researchers found that though lung cancer in general has dropped off in during the last 30 years, certain specific types of lung cancer have actually increased in rate.

One of the main reasons cited that may be behind the decreasing cancer rates is the similar decrease in smoking rates. Smoking accounts for up to 95 percent of all occurrences of lung cancer. Researchers were not able to single out any factors as absolutely being the force behind the decline, however.

The increased rate of specific types of lung cancers have encouraged researchers to call for more investigation. Squamos cell carcinomas, for example, have exhibited behaviors that researchers have found baffling.

Another form of lung cancer, adenocarcinomas, seem to be increasing in frequency. Adenocarcinomas are located on the outer portions of the lungs and are known to be a slow growing form of cancer. This type of cancer makes up nearly 40 percent of all lung cancer cases.

One suggestion given by researchers regarding the increase in adenocarcinomas is the method of smoking utilized by a smoker. Those who inhale very deeply expose more of their lung tissues to the carcinogenic tar found in cigarettes.

Yet another mystery uncovered by the scientists involved was the shift in rates among genders. Historically, men have comprised most of the diagnoses of lung cancer. These rates among men are decreasing, but the rates among women do not seem to be following suit. Now, the gender gap in lung cancer rates is noticeably less distinct.

Researchers don’t know why several of these factors have surfaced regarding the strange behavior of some aspects of lung cancer. They are calling for more research to help increase understanding about what is at play in the way the rates are currently being recorded.