The light is not utilized, which often exceeds 25% in some cases exceeded 50% (balloon-type lamps), we also have to pay. Obliges us to use higher power lamps are used as only a part thereof, the remainder being lost unnecessarily. This excess of consumption that must supply the power stations, is more fuel consumption and, consequently, higher emissions of polluting gases into the atmosphere, among other things responsible for the greenhouse effect is increasing the average temperature of our planet and acid rain that destroys forests.
Light falling directly from the lamp in our eyes has an intensity well above that comes reflected from the ground and the obstacles it presented, we see them doing worse since the opening of the pupils has closed to adjust to that light intensity.
This annoying glare produces eyestrain us, reduces our perception and, consequently, increases the risk of accidents, ie, reduce road safety.
It also adversely affects the nightlife of wildlife, especially to certain birds (well-known case is that of the offspring of shearwater in its first flight are disoriented and crash due to glare of stocks, hurting and dying in many times).
Intrusion into the privacy of persons:
By illuminating facades and windows with no light addressed to soil produce enough discomfort and facilitates the indiscretion of the passers-by.
Becomes difficult and obscure vision of the starry sky: This is a really unfortunate consequence for all and especially for the astronomer, as this wonderful spectacle, an inexhaustible source of knowledge and beauty, which undoubtedly is the vision of the rest of the Universe which we belong, has degraded so because of such contamination, until we have lost the habit of looking at the sky at night, after no longer get our attention.