Cities can meet many people's needs, such as convenience for work, shopping, medical care, education, entertainment, interpersonal communication, access to information and so on. However, with the development of society, the scale of the city is getting bigger and bigger, the population of the city is increasing, and the buildings in the city are getting taller and denser, and this convenience and comfort are slowly losing. Crowded population, traffic jam, lack of water and electricity, air pollution, noise pollution, piles of garbage, cross-flow of sewage, lack of green space, reinforced concrete everywhere, the urban environment is getting worse and worse, and the original comfort and convenience of the city are gone. People pay more and more attention to urban environmental problems and the impact of urban environment on health.
With the increasing scale of cities, on the one hand, there has been a "scale effect" of economic development in big cities, on the other hand, there has also been a "pollution effect" that we do not want to see. That is, due to the increase of urban population, the pollution caused by automobile exhaust has also become obviously stronger, the use of water, electricity, gas and coal has increased obviously, and the pollution caused by urban dust has become more serious.
First, automobile exhaust emissions have increased sharply in busy streets, and thick light blue smoke has been sprayed from the tail of vehicles, commonly known as automobile exhaust. This kind of gas emission not only smells strange, but also makes people dizzy and sick, which affects people's health.
The pollution caused by automobile exhaust is obviously related to respiratory diseases, various cancers (especially lung cancer has risen to the first place in some big cities), skin diseases, cardiovascular diseases and nervous system diseases.
Now there are more and more cars in the city, and the problem of urban air pollution caused by automobile exhaust is becoming more and more prominent. In some cities, automobile exhaust has become the main pollution source of urban air.
Automobile exhaust contains hundreds of different compounds, among which harmful pollutants are solid suspended particles, carbon monoxide, carbon dioxide, hydrocarbons, nitrogen oxides, lead and sulfur oxides. The harmful exhaust gas emitted by a car in a year is three times larger than its own weight. The British Association for Air Cleanliness and Environmental Protection has published a research report saying that compared with the victims of traffic accidents, the number of people who die from air pollution in Britain is 10 times more each year.
Lead is used as an explosion-proof agent. This kind of gasoline is called leaded gasoline. Leaded gasoline makes the tail gas emitted by automobiles contain high concentration of lead, which is harmful to human health. In view of this, China began to use unleaded gasoline in 2000, and the corresponding tetraethyl lead was replaced by a series of new gasoline explosion-proof agents. In China, unleaded gasoline means that the lead content is 0? 0 13g/L gasoline. Therefore, unleaded gasoline is not zero lead gasoline, so automobile exhaust still contains a small amount of lead. Rural residents generally inhale about 1 microgram of lead from the air every day; The daily intake of lead by urban residents, especially those on both sides of the street, will greatly exceed this value. More than 60% of lead in urban air comes from the burning of leaded gasoline in automobiles. Excessive lead content in human body will cause cardiovascular diseases and affect the functions of important organs such as liver and kidney and nervous system. Because of the high ratio of lead to dust, it usually accumulates in the air at a height of about 1 m, so it poses the greatest threat to children.
At present, the new explosion-proof agents to replace tetraethyl lead in unleaded gasoline mainly include aromatic hydrocarbon, methyl tert-butyl ether (MTBE), triethyl butyl ether, tripentyl methyl ether, manganese carbonyl (MMT), alcohols and so on. , of which MTBE is the largest.
The main harmful components in automobile exhaust are unburned or incompletely burned methane, nitrogen oxides, carbon monoxide, carbon dioxide, sulfur dioxide, H2S and trace amounts of aldehydes, phenols, peroxides, organic acids and lead and phosphorus pollution caused by gasoline containing lead and phosphorus. Among them, carbon monoxide, hydrocarbons, nitrogen oxides, lead compounds and particulate matter are the most harmful to people.
The particulate matter in automobile exhaust contains a strong carcinogen benzo (a) pyrene. Under normal circumstances, 1 g particulate matter contains about 70 micrograms of benzo (a) pyrene, and every 1 kg gasoline can produce 30 milligrams of benzo (a) pyrene. When the concentration of benzo (a) pyrene in the air reaches 0? 0 12 μ g/m3, the number of people suffering from lung cancer will increase significantly.
Second, the consumption of urban resources is huge. Of the more than 600 cities in China, 400 are short of water. With the shortage of surface water, people turn to groundwater more and more, resulting in the imbalance between groundwater exploitation and recharge, which leads to the continuous decline of groundwater level and a series of serious consequences, such as the abandonment of wells, seawater intrusion, ground subsidence, crack collapse, vegetation dying and so on. Urban water pollution and the resulting shortage of water resources have reached a critical moment affecting urban survival.
The city is like a big pollution body. After the clean river flows through the city, it becomes a dirty and smelly sewage river. The city poured waste water into the river; The city threw its waste into the river; The city washed its dirt into the river. With the development of China's industry and the increase of urban population, the discharge of industrial wastewater and domestic sewage increases sharply, and the urban sewage treatment capacity can't keep up with the development, which leads to serious pollution and eutrophication of most urban sections of rivers and urban lakes, triggering urban water crisis and threatening people's life, health and safety.
The demand for electricity in cities continues to grow. Every summer, high temperature and other factors cause the peak of electricity consumption, and many cities frequently have electricity emergencies. The improvement of power supply capacity is limited, the growth of installed power generation is much lower than the rapid growth of electricity consumption, and the utilization hours of power generation equipment continue to climb. Cross-regional power transmission has greatly increased, and national networking has played an obvious role in alleviating the tension between supply and demand in some areas. The supply of thermal coal is obviously insufficient and the price continues to rise. Long-term overload operation of generator sets makes it difficult to arrange maintenance of generator sets, and even leads to an increase in unplanned outages. In many provinces and cities in China, due to the difficulty in arranging unit maintenance and non-maintenance, unplanned outages have increased, which directly leads to or aggravates the power shortage situation.
The demand for natural gas and coal in cities is also huge. Whether it is gas or coal, it will "help" the aggravation of urban air pollution, especially in cities dominated by coal, where the increase of dust and sulfur oxides in the air is very obvious.
Third, the harm of urban dust is not shallow. Dust can be divided into falling dust and floating dust. The falling dust particles are larger, the particle size is above 10 micron, and most of them can fall to the ground quickly, which are small carbon particles with incomplete combustion, that is, black smoke that people see. The floating dust particles are smaller, with the particle size below 10 micron, and a considerable part of them are smaller than bacteria and float in the air for a long time. Particles with a particle size of 5- 10 micron can enter the respiratory system and are discharged by nasal hairs and respiratory mucus due to inertial force. Particles smaller than 10 micron adhere to the surface of upper respiratory tract and are discharged with sputum due to gas diffusion. However, the floating dust of half a micron to 5 microns can directly reach the lung cells and deposit, enter the blood and be sent to the whole body, and accumulate in various parts of the body, causing diseases.
Dust in the city can be composed of road dust brought by the wind after driving, dust emitted by domestic coal burning, smoke and dust emitted by industrial boilers, solid suspended particles emitted by automobile exhaust, etc. Dust also contains a variety of metal particles and gases, such as lead, mercury, cadmium, chromium, vanadium, iron and their oxides, which have a catalytic effect and can promote its adsorption of harmful gases such as sulfur dioxide and nitrogen oxides. Floating dust can also adsorb hydrocarbons such as benzopyrene, which is highly carcinogenic.
In addition, if people spit everywhere, all kinds of bacteria in sputum, especially mycobacterium tuberculosis, will also float into the air with dust, and may be infected with tuberculosis after inhalation.
Urban air pollution can be enriched in the atmosphere under the meteorological conditions such as no wind, low wind speed and inversion temperature, which is not conducive to dilution and diffusion, thus aggravating the degree of air pollution and greatly enhancing the harm to public safety.
In addition, the problems of noise pollution, light pollution, electromagnetic pollution and heat pollution in cities are becoming increasingly acute.