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   20804篇
  免费   311篇
  国内免费   334篇
安全科学   645篇
废物处理   947篇
环保管理   2803篇
综合类   3464篇
基础理论   5160篇
环境理论   11篇
污染及防治   5760篇
评价与监测   1318篇
社会与环境   1208篇
灾害及防治   133篇
  2023年   122篇
  2022年   272篇
  2021年   271篇
  2020年   221篇
  2019年   187篇
  2018年   410篇
  2017年   347篇
  2016年   568篇
  2015年   409篇
  2014年   629篇
  2013年   1709篇
  2012年   824篇
  2011年   1086篇
  2010年   820篇
  2009年   882篇
  2008年   1023篇
  2007年   1069篇
  2006年   894篇
  2005年   715篇
  2004年   713篇
  2003年   656篇
  2002年   618篇
  2001年   692篇
  2000年   552篇
  1999年   336篇
  1998年   221篇
  1997年   238篇
  1996年   227篇
  1995年   286篇
  1994年   224篇
  1993年   215篇
  1992年   180篇
  1991年   186篇
  1990年   184篇
  1989年   187篇
  1988年   158篇
  1987年   138篇
  1986年   159篇
  1985年   147篇
  1984年   190篇
  1983年   149篇
  1982年   175篇
  1981年   157篇
  1980年   132篇
  1979年   152篇
  1978年   94篇
  1977年   100篇
  1975年   93篇
  1974年   94篇
  1972年   97篇
排序方式: 共有10000条查询结果,搜索用时 562 毫秒
271.
The aim of this study was to determine whether there is increased leakage of neuron-specific enolase (NSE) and S-100 protein into amniotic fluid in pregnancies with neural tube defects, since both these proteins are produced by neural tissue, and to compare the value of these substances for detecting such defects with that of the more conventional techniques of alpha-fetoprotein (AFP) and acetylcholinesterase (AChE) gel electrophoresis. Amniotic samples from 25 mid-pregnancies (15–17 weeks' gestation) with neural tube defects (14 with open spina bifida and 11 with anencephaly) and from seven mid-pregnancies with abdominal wall defects were compared with a control material consisting of 80 amniotic fluid samples from 80 consecutive mid-pregnancy amniocenteses, with normal karyotypes and AFP concentrations. All of the above cases of abnormalities were primarily detected through increased AFP levels in the amniotic fluid. Amniotic fluid samples from 13 pregnancies with fetuses with autosomal chromosomal abnormalities and seven amniotic fluid samples contaminated with blood were also included in the investigation. It is concluded from the results that the conventional AFP assay combined with AChE gel electrophoresis is the best method for screening amniotic fluid for neural tube defects and defects of the abdominal wall. Neither NSE nor S-100 assay alone proved to be superior for the detection of these cases in mid-trimester amniotic fluid. The S-100 assay, however, could give additional information in cases where AChE gel electrophoresis is not decisive; for example, in samples contaminated with blood.  相似文献   
272.
Using a modified procedure by Solomons and Styner (1969), an evaluation of inorganic pyrophosphate (PPi) was performed on the amniotic fluid of two fetuses at risk for osteogenesis imperfecta (OI) at 14½ weeks gestation. The parents of both cases had a previous child with OI, Type II. The normal control group at 14–16 weeks gestation had PPi values ranging from 22.0–59.2 ug/100 ml, with a mean of 38.6±9.51 ug/100 ml. In each at-risk fetus, the amniotic fluid PPi value was within normal range. The first baby was born phenotypically normal at term. Intrauterine radiographic and fetal sonograms were done on the second fetus at approximately 19 weeks gestation. Both showed evidence of OI, Type II. The pregnancy was terminated at 21 weeks. Radiologic studies of the aborted fetus were consistent with OI, Type II. Our results indicate that the evaluation of PPi levels in amniotic fluid is not the method of choice for prenatal diagnosis of IO.  相似文献   
273.
引言 墨西哥城大都市区(MCMA)位于被称作"墨西哥谷(Valle deMexico)"的天然围合的盆地里,海拔2240m.它下辖联邦区和墨西哥州的34个自治市.墨西哥城是墨西哥的首都,位于联邦区(DistritoFederal,DF),四面环山,海拔高度超过5000m.墨西哥州是墨西哥人口最稠密的地区(据报道有1300万居民),人口仅次于它的是墨西哥城,大约有860万人[1].  相似文献   
274.
275.
Foreword     
Mitigation and Adaptation Strategies for Global Change -  相似文献   
276.
This paper investigates the current treatment of environmental issues by two companies within the large made-to-order sector. Fifty questionnaires were issued to the design team of each company to assess current practice in the integration of environmental issues with design activities, awareness of environmental issues, awareness of existing environmental design tools and the preferred format for a potential clean design tool.  相似文献   
277.
Maternal serum alpha-fetoprotein (AFP) and beta-human chorionic gonadotropin (βhCG) measurements taken prior to chorionic villus sampling (CVS) in 21 patients who subsequently miscarried were compared with measurements in a control group of 113 patients with uneventful pregnancies. Patients with AFP levels of 10 iu/ml or more prior to the CVS had a 4·3 times greater risk of miscarriage (95 per cent confidence interval 1·3–13·6). AFP levels obtained 1 week after the CVS in the 13 patients with late miscarriages were higher than in the control group (P = 0·06). Patients miscarrying had a greater rise in AFP (P = 0·06) and a greater fall in βhCG levels (P = 0·04) following the CVS procedure, compared with the control subjects. Each 10-unit change in the difference between AFP or βhCG levels prior to and 1 week following the CVS was associated with a significantly increased risk for late miscarriage. Elevated maternal serum AFP levels early in pregnancy and changes in AFP and βhCG levels following CVS may predict an increased risk for subsequent miscarriage.  相似文献   
278.
279.
280.
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