In 2004, the FAA introduced the "Light-sport aircraft" category, which resembles some other countries microlight categories. Ultralight aviation is represented by the United States Ultralight Association (USUA), which acts as the US aeroclub representative to the Fédération Aéronautique Internationale. Types
There are several categories of aircraft which qualify as ultralights in some countries:

Fixed-wing aircraft: traditional airplane-style designs. Weight-shift control trike: use a hang glider-style wing, below which is suspended a three-wheeled carriage which carries the engine and aviators. These aircraft are controlled by pushing against a horizontal control bar in roughly the same way as a hang glider pilot flies. Powered parachute: fuselage-mounted engines with parafoil wings, which are wheeled aircraft. Powered paraglider: backpack engines with parafoil wings, which are foot-launched. Powered hang glider: motorized foot-launched hang glider harness. Autogyro: rotary wing with fuselage-mounted engine, a gyrocopter is different from a helicopter in that the rotating wing is not powered, the engine provides forward thrust and the airflow through the rotary blades causes them to autorotate or "spin up" thereby creating lift. Helicopter: there are a number of single-seat and two-place helicopters which fall under the microlight categories in countries such as New Zealand. However, few helicopter designs fall within the more restrictive ultralight category defined in the United States of America. Hot air balloon: there are numerous ultralight hot air balloons in the US, and several more have been built and flown in France and Australia in recent years.
It is hypothesized that for people with dandruff, skin cells may mature and be shed in 2–7 days, as opposed to around a month in people without dandruff. The result is that dead skin cells are shed in large, oily clumps, which appear as white or grayish flakes on the scalp, skin and clothes. According to one study, dandruff has been shown to be possibly the result of three factors:
Skin oil, commonly referred to as sebum or sebaceous secretions
The metabolic by-products of skin micro-organisms (most specifically Malassezia yeasts)
Individual susceptibility and allergy sensitivity. Microorganisms
Older literature cites the fungus Malassezia furfur (previously known as Pityrosporum ovale) as the cause of dandruff. While this species does occur naturally on the skin surface of people both with and without dandruff, in 2007, it was discovered that the responsible agent is a scalp specific fungus, Malassezia globosa, that metabolizes triglycerides present in sebum by the expression of lipase, resulting in a lipid byproduct: oleic acid. During dandruff, the levels of Malassezia increase by 1.5 to 2 times its normal level. Oleic acid penetrates the top layer of the epidermis, the stratum corneum, and evokes an inflammatory response in susceptible people which disturbs homeostasis and results in erratic cleavage of stratum corneum cells. Seborrhoeic dermatitis
In seborrhoeic dermatitis, redness and itching frequently occur around the folds of the nose and eyebrow areas, not just the scalp. Dry, thick, well-defined lesions consisting of large, silvery scales may be traced to the less common condition of scalp psoriasis.