首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   16360篇
  免费   172篇
  国内免费   139篇
安全科学   449篇
废物处理   627篇
环保管理   2291篇
综合类   2572篇
基础理论   4463篇
环境理论   4篇
污染及防治   4302篇
评价与监测   1007篇
社会与环境   852篇
灾害及防治   104篇
  2022年   111篇
  2021年   129篇
  2020年   105篇
  2019年   149篇
  2018年   224篇
  2017年   221篇
  2016年   355篇
  2015年   241篇
  2014年   357篇
  2013年   1267篇
  2012年   442篇
  2011年   632篇
  2010年   539篇
  2009年   519篇
  2008年   690篇
  2007年   692篇
  2006年   659篇
  2005年   506篇
  2004年   587篇
  2003年   525篇
  2002年   482篇
  2001年   703篇
  2000年   471篇
  1999年   285篇
  1998年   242篇
  1997年   217篇
  1996年   228篇
  1995年   247篇
  1994年   271篇
  1993年   219篇
  1992年   247篇
  1991年   223篇
  1990年   254篇
  1989年   238篇
  1988年   193篇
  1987年   173篇
  1986年   158篇
  1985年   167篇
  1984年   192篇
  1983年   180篇
  1982年   185篇
  1981年   179篇
  1980年   137篇
  1979年   153篇
  1978年   131篇
  1977年   117篇
  1975年   118篇
  1974年   115篇
  1973年   110篇
  1972年   133篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
701.
In a case of fetal heart failure caused by endocardial fibroelastosis, prenatal echocardiography clearly demonstrated; a thickened endocardium. We therefore suggest that an abnormal endocardium may be detected in utero by ultrasound, thus representing an important clue in the differential diagnosis of fetal nonimmune hydrops and in the evaluation of pregnancies at risk for endocardial fibroelastosis.  相似文献   
702.
703.
704.
705.
Amniocentesis and subsequent tests are reported on a fetus conceived of a rare mating type: its mother has an intermediate level of β hexosaminidase A (HEX A), characteristic of carriers of Tay-Sachs disease (TSD), while the father suffers from an adult-onset GM2 ganglio-sidosis (AOG) with severe HEX A deficiency. Activity of HEX A in the cultured fetal cells was very low when measured by the heat-inactivation method, thus showing the typical biochemical phenotype of TSD fetuses. However, upon separation of HEX isozymes by ion exchange chromatography, residual HEX A (17 per cent of total HEX) was demonstrated. Also in contrast to TSD fetuses, this fetus' fibroblasts were able to synthesize the precursor of a chains of HEX, and ultrastructural examination of its brain revealed few atypical lamellar bodies, unlike those found in TSD fetuses of the same gestational age. It is therefore concluded that the fetus was not affected with TSD, but rather with AOG.  相似文献   
706.
We have documented the presence of five mitochondrial enzymes in samples of chorionic villus tissue and measured the levels of activity. Three of the enzymes catalyse biotindependent reactions. These are propionyl-CoA carboxylase, 3-methylcrotonyl-CoA carboxylase and pyruvate carboxylase. the other enzymes. 4-aminobutyric acid aminotransferase and succinic semialdehyde dehydrogenase, are involved inthe degradation of the central inhibitory neurotransmitter GABA. Distinct diseases in whichthere is deficiency of each of these enzymes have been documented in man. Significant levels of activity were observed for all five enzymes in chorionic villus tissue. This methodology should permit early prenatal diagnosis of deficiencies of these enzymes by chorionic villus biopsy in the first trimester.  相似文献   
707.
A case of spontaneous abortion after transcervical CVS is presented. Despite no evidence of bacteria in the vagina and cervix prior to sampling and the prophylactic use of Metronidazole, pathological evidence of chorioamnionitis was found. The implications of this are discussed.  相似文献   
708.
Metal Pollution Assessment of Sediment and Water in the River Hindon, India   总被引:7,自引:0,他引:7  
The metal pollution in water and sediment of the River Hindon in western Uttar Pradesh (India) was assessed for Cd, Cr, Cu, Fe, Mn, Ni, Pb and Zn. The metal concentrations in water showed wide temporal variation compared with bed sediment because of variability in water discharge and variations in suspended solid loadings. Metal concentrations in bed sediments provided a better evaluation of the degree and the extent of contamination in the aquatic environment, Santagarh and Atali being the most polluted sites of the river. The ratio of heavy metals to conservative elements (Fe, Al, etc.) may reveal the geochemical imbalances due to the elevated metal concentrations normally attributed to anthropogenic sources. Metal/Al ratios for the bed sediments of the river Hindon were used to determine the relative mobility and general trend of relative mobility occurred Fe > Mn > Zn > Cr > Ni > Pb > Cu > Cd.  相似文献   
709.
710.
The Catskill/Delaware reservoirs supply 90% of New York City’s drinking water. The City has implemented a series of watershed protection measures, including land acquisition, aimed at preserving water quality in the Catskill/Delaware watersheds. The objective of this study was to examine how relationships between landscape and surface water measurements change between years. Thirty-two drainage areas delineated from surface water sample points (total nitrogen, total phosphorus, and fecal coliform bacteria concentrations) were used in step-wise regression analyses to test landscape and surface-water quality relationships. Two measurements of land use, percent agriculture and percent urban development, were positively related to water quality and consistently present in all regression models. Together these two land uses explained 25 to 75% of the regression model variation. However, the contribution of agriculture to water quality condition showed a decreasing trend with time as overall agricultural land cover decreased. Results from this study demonstrate that relationships between land cover and surface water concentrations of total nitrogen, total phosphorus, and fecal coliform bacteria counts over a large area can be evaluated using a relatively simple geographic information system method. Land managers may find this method useful for targeting resources in relation to a particular water quality concern, focusing best management efforts, and maximizing benefits to water quality with minimal costs.The United States Environmental Protection Agency through its Office of Research and Development funded and managed the research described here. It has been subjected to Agency’s administrative review and approved for publication as an EPA document.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号