Assalamualaikum. Hye ols ! Han rasa minggu nih han stress sekali ! Uwaaaaaaaaaaaaaaaaa. Raso nok nepik jah sekuat- kuat nyo. Hemph. Okela. Han bananyo stress ko bio 310. Han xtau bakpo. Han raso sem niyh han xdop semangat lah mano nak belajar bio. Han dok pehe ge satu gapo hok dok blajar. Tgk, air mata nak kuar dah. Sensitif giler mata han neh. Tulong. Sapo buleh ajar han ? Tulong sgt2. Han dok rok doh nak tanggung perasaan niyh. Yg paling han serabut kepala, bio niyh, paper muet sabtu niyh, tiket nak mari ganu lepas rayer niyh. Owh, sgt stress. Oke, dah lah. Han bena nok kabo pasal cuaca loniyh
Korg perasan x yg bahawa bumi kita ni dah tua. Masa berlalu dgn cepat. Cuaca pon panas giler. Kol 9 pg tuh mcm kol 12 masa zmn skoloh dulu. Masya- ALLAH. Perit nyo raso kulit. Panas gitu. Meme menusuk sapa ko tule han raso. Hemph.
| Ultraviolet (UV) irradiation present in sunlight is an environmental human carcinogen. The toxic effects of UV from natural sunlight and therapeutic artificial lamps are a major concern for human health. The major acute effects of UV irradiation on normal human skin comprise sunburn inflammation erythema, tanning, and local or systemic immunosuppression. | ” |
| |
— Matsumura and Ananthaswamy , (2004)[22]
|
UVA, UVB, and UVC can all damage
collagen fibers and, therefore, accelerate aging of the skin. Both UVA and UVB destroy vitamin A in skin, which may cause further damage.
[23] In the past, UVA was considered not harmful or less harmful, but today it is known it can contribute to skin cancer via
indirect DNA damage (free radicals and reactive oxygen species). It penetrates deeply, but it does not cause
sunburn. UVA does not
damage DNA directly like UVB and UVC, but it can generate highly reactive chemical intermediates, such as hydroxyl and oxygen radicals, which in turn can damage DNA. Accordingly the DNA damage caused indirectly to skin by UVA consists mostly of single-strand breaks in DNA, while the damage caused by UVB includes direct formation of
thymine dimers or other
pyrimidine dimers, and double-strand DNA breakage.
[24] UVA is immunosuppressive for the entire body (accounting for a large part of the immunosuppressive effects of sunlight exposure), and UVA is mutagenic for basal cell keratinocytes in skin.
[25]
Because UVA does not cause reddening of the skin (erythema), it is not measured in the usual types of
SPF testing.
[26] There is no good clinical measurement for blockage of UVA radiation, but it is important for
sunscreento block both UVA and UVB. Some scientists blame the absence of UVA filters in
sunscreens for the higher melanoma risk found for sunscreen users.
[27]
The reddening of the skin due to the action of sunlight depends both on the amount of sunlight and on the sensitivity of the skin ("erythemal action spectrum") over the UV spectrum.
UVB light can cause direct DNA damage. As noted above UVB radiation
excites DNA molecules in skin cells, causing aberrant
covalent bonds to form between adjacent
cytosine bases, producing a
dimer. When DNA polymerase comes along to replicate this strand of DNA, it reads the dimer as "AA" and not the original "CC". This causes the DNA replication mechanism to add a "TT" on the growing strand. This
mutation can result in
cancerous growths, and is known as a "classical C-T mutation". The mutations caused by the direct DNA damage carry a UV signature mutation that is commonly seen in skin
cancers. The
mutagenicity of UV radiation can be easily observed in
bacterial cultures. This cancer connection is one reason for concern about
ozone depletion and the ozone hole.
As a defense against UV radiation, the type and amount of the brown pigment
melanin in the skin increases when exposed to moderate (depending on
skin type) levels of radiation; this is commonly known as a
sun tan. The purpose of melanin is to absorb UV radiation and dissipate the energy as harmless heat, blocking the UV from damaging skin tissue. UVA gives a quick tan that lasts for days by oxidizing melanin that was already present, and it triggers the release of the
melanin from melanocytes. However, because this process does not increase the total amount of melanin, a UVA-produced tan is largely cosmetic and does not protect against either sun burn or UVB-produced DNA damage or cancer.
[28]
By contrast, UVB yields a slower tan that requires roughly two days to develop, because the mechanism of UVB tanning is to stimulate the body to produce more melanin. However, the production of melanin by UV, called
melanogenesis, requires direct DNA damage by UVB to initiate. The photochemical properties of melanin make it an excellent
photoprotectant from both UVA and UVB. Older and more widespread sunscreen chemicals cannot dissipate the energy of the excited state as efficiently as melanin, and, therefore, the penetration of these sunscreen ingredients into the lower layers of the skin may increase the amount of
free radicals and reactive oxygen species (
ROS).
[29] In recent years, improved filtering substances have come into use in commercial sunscreen lotions that do not significantly degrade or lose their capacity to protect the skin as the exposure time increases (
photostable substances).
[30]
Sunscreen prevents the
direct DNA damage that causes sunburn, by blocking of UVB. As such, most of these products contain an
SPF rating that indicates how well they block UVB as a measure of their effectiveness (SPF is therefore also called UVB-PF, for "UVB protection factor").
[31] This rating, however, offers no data about protection against UVA, exposure to which does not lead to sunburn but is still harmful since it causes indirect UV DNA damage and is also (along with UVB and UVC) considered carcinogenic. In the US, the
Food and Drug Administration is considering adding a star rating system to show UVA protection (also known as UVA-PF). A similar system is already used in some European countries.
[32] Some sunscreen lotions now include compounds such as
titanium dioxide, which helps protect against UVA rays. Other UVA blocking compounds found in sunscreen include
zinc oxide and
avobenzone.
[edit] Sunscreen safety debate
Image of a man's face with sunscreen on the left, in visible
(left)and UV light, demonstrating how sunscreen protects against UV exposure. The side of the face with sunscreen is darker, showing that the sunscreen absorbs the UV light.
Medical organizations recommend patients protect themselves from UV radiation by using sunscreen. Five sunscreen ingredients have been shown to protect mice against skin tumors (see
sunscreen).
However, some sunscreen chemicals produce potentially harmful substances if they are illuminated while in contact with living cells.
[33][34][35] The amount of sunscreen that penetrates through the
stratum corneum may be large enough to cause damage. In one study of sunscreens, the authors write:
[36]
The question whether UV filters acts on or in the skin has so far not been fully answered. Despite the fact that an answer would be a key to improve formulations of sun protection products, many publications carefully avoid addressing this question.
In an experiment by Hanson et al. published in 2006, the amount of harmful
reactive oxygen species (ROS) was measured in untreated and in sunscreen treated skin. In the first 20 minutes, the film of sunscreen had a protective effect and the amount of ROS was smaller. After 60 minutes, however, the amount of absorbed sunscreen was so high, the amount of ROS was higher in the sunscreen treated skin than in the untreated skin.
[29]
Such effects can be avoided by using newer generations of filter substances or combinations that maintain their UV protective properties even after several hours of solar exposure. Sunscreen products containing photostable filters like
drometrizole trisiloxane,
bisoctrizole, or
bemotrizinol have been available for many years throughout the world, but are not yet available in the U.S., whereas another high-quality filter,
ecamsule, has also been available in the U.S. since 2006.
[30]
Oke, ni han amek dr cik wikipedia. Just sharing this. Agak2 apa akan jd kat bumi kita niyh 2,3 tahun lg. Kalau semua org tau mcm mane nk kurgkan bnda niyh, mesty senang hidup. Tp satu bumi kot. Hemph. Okela, han nak tunggu asar jewh bena. Han nak study bio lg. Gud bye n pray for my muet test !
