When Henry Ford's model T rolled out of the factory the thought that one day cars would have tinted windows was not even a consideration. Beyond the windshield, there were no other windows on that model. Over time, cars and their windows have evolved to meet the demands of the driver. Tinted car windows had traditionally been used in police vehicles and limos to conceal their occupants. Eventually tinted windows were embraced by average drivers as fashionable, and cool. We see them on most SUVs and small cars owned by teenagers.

September 11, 2001 changed all of that. In order for law enforcement officials to be able to see into motor vehicles as they drove by, the tinted window had to go. Consequently, in many states car windows must have at least a 70% light transmission rate to be legal. As with most laws, medical conditions generally supersede the laws requirements. In this case there is one, and only one condition that permits car windows to be tinted beyond 70%. Individuals with this disorder are required to have a doctor's note with them all the time while driving in case they are stopped by law enforcement.

The medical condition that permits tinted windows is called Xeroderma Pigmentosum (XP). It is a very rare, recessive, inherited disease that significantly increases an individual’s sensitivity to sun light. It results in premature skin aging, and most often skin cancers. Even a small amount of UV light will cause severe sun burns that have a very high rate of malignancy.

The physiology of the disease is quite simple. Our skin is constantly being damaged by sun exposure, and needs to be repaired. Individuals with XP have a defect in their DNA repair system. Nucleotide excision enzymes that are needed to fix the sun damage are mutated, and unable to repair the damaged skin. As a result there is an alteration of the DNA in the damaged cells leading to mutations. This in turn results in the formation of cancerous cells.

The disease generally has 3 stages. The first appears about 6 months after birth. While the skin looks normal, exposed areas to the sun will get very red and become scaly with irregular spots and freckles. These changes will spread to the neck and legs even if they are not exposed to the sun. The spots may go away in the winter, but will reappear in the spring. The second stage is characterized by Poikiloderma. This level demonstrates thinning of the skin, light and dark patches and irregular web like blood spots and vessels that can be seen under the skin. The third stage is Solar Keratoses, and skin cancers. This stage may occur as early as 5-6 years old if there is sun exposure. The most common cancers seen are basal cell carcinoma, squamous cell carcinoma, and melanoma.

Ocular problems will occur in approximately 80% of these patients. There is extreme photosensitivity, and irritated, blood shot and cloudy eyes. Frequently, conjunctivitis occurs. In addition, lesions on their eye lids and surrounding ocular areas have been noted.

The need for dark, tinted windows is clearly justified in this case. The flexibility in this law allows individuals who otherwise would be unable to drive, to drive.